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![Nicolas Saenz Julienne](/assets/img/avatar_default.png)
The thread pool regulates itself: when idle, it kills threads until empty, when in demand, it creates new threads until full. This behaviour doesn't play well with latency sensitive workloads where the price of creating a new thread is too high. For example, when paired with qemu's '-mlock', or using safety features like SafeStack, creating a new thread has been measured take multiple milliseconds. In order to mitigate this let's introduce a new 'EventLoopBase' property to set the thread pool size. The threads will be created during the pool's initialization or upon updating the property's value, remain available during its lifetime regardless of demand, and destroyed upon freeing it. A properly characterized workload will then be able to configure the pool to avoid any latency spikes. Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Acked-by: Markus Armbruster <armbru@redhat.com> Message-id: 20220425075723.20019-4-nsaenzju@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
413 lines
12 KiB
C
413 lines
12 KiB
C
/*
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* Event loop thread
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*
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* Copyright Red Hat Inc., 2013, 2020
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*
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* Authors:
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* Stefan Hajnoczi <stefanha@redhat.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*
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*/
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#include "qemu/osdep.h"
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#include "qom/object.h"
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#include "qom/object_interfaces.h"
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#include "qemu/module.h"
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#include "block/aio.h"
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#include "block/block.h"
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#include "sysemu/event-loop-base.h"
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#include "sysemu/iothread.h"
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#include "qapi/error.h"
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#include "qapi/qapi-commands-misc.h"
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#include "qemu/error-report.h"
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#include "qemu/rcu.h"
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#include "qemu/main-loop.h"
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typedef ObjectClass IOThreadClass;
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DECLARE_CLASS_CHECKERS(IOThreadClass, IOTHREAD,
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TYPE_IOTHREAD)
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#ifdef CONFIG_POSIX
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/* Benchmark results from 2016 on NVMe SSD drives show max polling times around
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* 16-32 microseconds yield IOPS improvements for both iodepth=1 and iodepth=32
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* workloads.
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*/
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#define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
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#else
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#define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
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#endif
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static void *iothread_run(void *opaque)
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{
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IOThread *iothread = opaque;
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rcu_register_thread();
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/*
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* g_main_context_push_thread_default() must be called before anything
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* in this new thread uses glib.
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*/
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g_main_context_push_thread_default(iothread->worker_context);
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qemu_set_current_aio_context(iothread->ctx);
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iothread->thread_id = qemu_get_thread_id();
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qemu_sem_post(&iothread->init_done_sem);
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while (iothread->running) {
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/*
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* Note: from functional-wise the g_main_loop_run() below can
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* already cover the aio_poll() events, but we can't run the
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* main loop unconditionally because explicit aio_poll() here
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* is faster than g_main_loop_run() when we do not need the
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* gcontext at all (e.g., pure block layer iothreads). In
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* other words, when we want to run the gcontext with the
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* iothread we need to pay some performance for functionality.
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*/
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aio_poll(iothread->ctx, true);
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/*
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* We must check the running state again in case it was
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* changed in previous aio_poll()
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*/
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if (iothread->running && qatomic_read(&iothread->run_gcontext)) {
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g_main_loop_run(iothread->main_loop);
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}
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}
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g_main_context_pop_thread_default(iothread->worker_context);
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rcu_unregister_thread();
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return NULL;
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}
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/* Runs in iothread_run() thread */
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static void iothread_stop_bh(void *opaque)
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{
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IOThread *iothread = opaque;
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iothread->running = false; /* stop iothread_run() */
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if (iothread->main_loop) {
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g_main_loop_quit(iothread->main_loop);
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}
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}
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void iothread_stop(IOThread *iothread)
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{
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if (!iothread->ctx || iothread->stopping) {
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return;
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}
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iothread->stopping = true;
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aio_bh_schedule_oneshot(iothread->ctx, iothread_stop_bh, iothread);
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qemu_thread_join(&iothread->thread);
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}
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static void iothread_instance_init(Object *obj)
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{
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IOThread *iothread = IOTHREAD(obj);
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iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
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iothread->thread_id = -1;
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qemu_sem_init(&iothread->init_done_sem, 0);
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/* By default, we don't run gcontext */
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qatomic_set(&iothread->run_gcontext, 0);
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}
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static void iothread_instance_finalize(Object *obj)
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{
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IOThread *iothread = IOTHREAD(obj);
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iothread_stop(iothread);
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/*
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* Before glib2 2.33.10, there is a glib2 bug that GSource context
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* pointer may not be cleared even if the context has already been
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* destroyed (while it should). Here let's free the AIO context
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* earlier to bypass that glib bug.
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*
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* We can remove this comment after the minimum supported glib2
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* version boosts to 2.33.10. Before that, let's free the
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* GSources first before destroying any GMainContext.
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*/
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if (iothread->ctx) {
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aio_context_unref(iothread->ctx);
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iothread->ctx = NULL;
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}
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if (iothread->worker_context) {
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g_main_context_unref(iothread->worker_context);
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iothread->worker_context = NULL;
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g_main_loop_unref(iothread->main_loop);
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iothread->main_loop = NULL;
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}
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qemu_sem_destroy(&iothread->init_done_sem);
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}
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static void iothread_init_gcontext(IOThread *iothread)
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{
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GSource *source;
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iothread->worker_context = g_main_context_new();
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source = aio_get_g_source(iothread_get_aio_context(iothread));
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g_source_attach(source, iothread->worker_context);
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g_source_unref(source);
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iothread->main_loop = g_main_loop_new(iothread->worker_context, TRUE);
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}
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static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
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{
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IOThread *iothread = IOTHREAD(base);
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ERRP_GUARD();
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if (!iothread->ctx) {
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return;
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}
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aio_context_set_poll_params(iothread->ctx,
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iothread->poll_max_ns,
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iothread->poll_grow,
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iothread->poll_shrink,
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errp);
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if (*errp) {
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return;
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}
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aio_context_set_aio_params(iothread->ctx,
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iothread->parent_obj.aio_max_batch,
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errp);
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aio_context_set_thread_pool_params(iothread->ctx, base->thread_pool_min,
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base->thread_pool_max, errp);
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}
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static void iothread_init(EventLoopBase *base, Error **errp)
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{
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Error *local_error = NULL;
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IOThread *iothread = IOTHREAD(base);
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char *thread_name;
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iothread->stopping = false;
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iothread->running = true;
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iothread->ctx = aio_context_new(errp);
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if (!iothread->ctx) {
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return;
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}
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/*
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* Init one GMainContext for the iothread unconditionally, even if
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* it's not used
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*/
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iothread_init_gcontext(iothread);
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iothread_set_aio_context_params(base, &local_error);
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if (local_error) {
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error_propagate(errp, local_error);
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aio_context_unref(iothread->ctx);
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iothread->ctx = NULL;
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return;
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}
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/* This assumes we are called from a thread with useful CPU affinity for us
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* to inherit.
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*/
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thread_name = g_strdup_printf("IO %s",
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object_get_canonical_path_component(OBJECT(base)));
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qemu_thread_create(&iothread->thread, thread_name, iothread_run,
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iothread, QEMU_THREAD_JOINABLE);
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g_free(thread_name);
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/* Wait for initialization to complete */
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while (iothread->thread_id == -1) {
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qemu_sem_wait(&iothread->init_done_sem);
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}
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}
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typedef struct {
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const char *name;
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ptrdiff_t offset; /* field's byte offset in IOThread struct */
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} IOThreadParamInfo;
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static IOThreadParamInfo poll_max_ns_info = {
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"poll-max-ns", offsetof(IOThread, poll_max_ns),
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};
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static IOThreadParamInfo poll_grow_info = {
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"poll-grow", offsetof(IOThread, poll_grow),
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};
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static IOThreadParamInfo poll_shrink_info = {
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"poll-shrink", offsetof(IOThread, poll_shrink),
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};
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static void iothread_get_param(Object *obj, Visitor *v,
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const char *name, IOThreadParamInfo *info, Error **errp)
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{
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IOThread *iothread = IOTHREAD(obj);
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int64_t *field = (void *)iothread + info->offset;
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visit_type_int64(v, name, field, errp);
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}
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static bool iothread_set_param(Object *obj, Visitor *v,
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const char *name, IOThreadParamInfo *info, Error **errp)
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{
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IOThread *iothread = IOTHREAD(obj);
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int64_t *field = (void *)iothread + info->offset;
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int64_t value;
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if (!visit_type_int64(v, name, &value, errp)) {
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return false;
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}
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if (value < 0) {
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error_setg(errp, "%s value must be in range [0, %" PRId64 "]",
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info->name, INT64_MAX);
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return false;
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}
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*field = value;
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return true;
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}
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static void iothread_get_poll_param(Object *obj, Visitor *v,
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const char *name, void *opaque, Error **errp)
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{
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IOThreadParamInfo *info = opaque;
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iothread_get_param(obj, v, name, info, errp);
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}
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static void iothread_set_poll_param(Object *obj, Visitor *v,
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const char *name, void *opaque, Error **errp)
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{
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IOThread *iothread = IOTHREAD(obj);
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IOThreadParamInfo *info = opaque;
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if (!iothread_set_param(obj, v, name, info, errp)) {
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return;
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}
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if (iothread->ctx) {
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aio_context_set_poll_params(iothread->ctx,
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iothread->poll_max_ns,
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iothread->poll_grow,
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iothread->poll_shrink,
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errp);
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}
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}
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static void iothread_class_init(ObjectClass *klass, void *class_data)
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{
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EventLoopBaseClass *bc = EVENT_LOOP_BASE_CLASS(klass);
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bc->init = iothread_init;
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bc->update_params = iothread_set_aio_context_params;
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object_class_property_add(klass, "poll-max-ns", "int",
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iothread_get_poll_param,
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iothread_set_poll_param,
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NULL, &poll_max_ns_info);
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object_class_property_add(klass, "poll-grow", "int",
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iothread_get_poll_param,
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iothread_set_poll_param,
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NULL, &poll_grow_info);
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object_class_property_add(klass, "poll-shrink", "int",
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iothread_get_poll_param,
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iothread_set_poll_param,
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NULL, &poll_shrink_info);
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}
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static const TypeInfo iothread_info = {
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.name = TYPE_IOTHREAD,
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.parent = TYPE_EVENT_LOOP_BASE,
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.class_init = iothread_class_init,
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.instance_size = sizeof(IOThread),
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.instance_init = iothread_instance_init,
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.instance_finalize = iothread_instance_finalize,
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};
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static void iothread_register_types(void)
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{
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type_register_static(&iothread_info);
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}
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type_init(iothread_register_types)
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char *iothread_get_id(IOThread *iothread)
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{
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return g_strdup(object_get_canonical_path_component(OBJECT(iothread)));
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}
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AioContext *iothread_get_aio_context(IOThread *iothread)
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{
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return iothread->ctx;
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}
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static int query_one_iothread(Object *object, void *opaque)
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{
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IOThreadInfoList ***tail = opaque;
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IOThreadInfo *info;
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IOThread *iothread;
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iothread = (IOThread *)object_dynamic_cast(object, TYPE_IOTHREAD);
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if (!iothread) {
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return 0;
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}
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info = g_new0(IOThreadInfo, 1);
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info->id = iothread_get_id(iothread);
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info->thread_id = iothread->thread_id;
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info->poll_max_ns = iothread->poll_max_ns;
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info->poll_grow = iothread->poll_grow;
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info->poll_shrink = iothread->poll_shrink;
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info->aio_max_batch = iothread->parent_obj.aio_max_batch;
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QAPI_LIST_APPEND(*tail, info);
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return 0;
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}
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IOThreadInfoList *qmp_query_iothreads(Error **errp)
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{
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IOThreadInfoList *head = NULL;
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IOThreadInfoList **prev = &head;
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Object *container = object_get_objects_root();
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object_child_foreach(container, query_one_iothread, &prev);
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return head;
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}
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GMainContext *iothread_get_g_main_context(IOThread *iothread)
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{
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qatomic_set(&iothread->run_gcontext, 1);
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aio_notify(iothread->ctx);
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return iothread->worker_context;
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}
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IOThread *iothread_create(const char *id, Error **errp)
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{
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Object *obj;
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obj = object_new_with_props(TYPE_IOTHREAD,
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object_get_internal_root(),
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id, errp, NULL);
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return IOTHREAD(obj);
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}
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void iothread_destroy(IOThread *iothread)
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{
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object_unparent(OBJECT(iothread));
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}
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/* Lookup IOThread by its id. Only finds user-created objects, not internal
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* iothread_create() objects. */
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IOThread *iothread_by_id(const char *id)
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{
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return IOTHREAD(object_resolve_path_type(id, TYPE_IOTHREAD, NULL));
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}
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bool qemu_in_iothread(void)
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{
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return qemu_get_current_aio_context() == qemu_get_aio_context() ?
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false : true;
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}
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