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488c84acb4
When reviewing my attempt to refactor send_prepare(), Fabiano suggested we try out with dropping the mutex in multifd code [1]. I thought about that before but I never tried to change the code. Now maybe it's time to give it a stab. This only optimizes the sender side. The trick here is multifd has a clear provider/consumer model, that the migration main thread publishes requests (either pending_job/pending_sync), while the multifd sender threads are consumers. Here we don't have a lot of complicated data sharing, and the jobs can logically be submitted lockless. Arm the code with atomic weapons. Two things worth mentioning: - For multifd_send_pages(): we can use qatomic_load_acquire() when trying to find a free channel, but that's expensive if we attach one ACQUIRE per channel. Instead, keep the qatomic_read() on reading the pending_job flag as we do already, meanwhile use one smp_mb_acquire() after the loop to guarantee the memory ordering. - For pending_sync: it doesn't have any extra data required since now p->flags are never touched, it should be safe to not use memory barrier. That's different from pending_job. Provide rich comments for all the lockless operations to state how they are paired. With that, we can remove the mutex. [1] https://lore.kernel.org/r/87o7d1jlu5.fsf@suse.de Suggested-by: Fabiano Rosas <farosas@suse.de> Reviewed-by: Fabiano Rosas <farosas@suse.de> Link: https://lore.kernel.org/r/20240202102857.110210-24-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com> |
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| .. | ||
| block-dirty-bitmap.c | ||
| block.c | ||
| block.h | ||
| channel-block.c | ||
| channel-block.h | ||
| channel.c | ||
| channel.h | ||
| colo-failover.c | ||
| colo.c | ||
| dirtyrate.c | ||
| dirtyrate.h | ||
| exec.c | ||
| exec.h | ||
| fd.c | ||
| fd.h | ||
| file.c | ||
| file.h | ||
| global_state.c | ||
| meson.build | ||
| migration-hmp-cmds.c | ||
| migration-stats.c | ||
| migration-stats.h | ||
| migration.c | ||
| migration.h | ||
| multifd-zlib.c | ||
| multifd-zstd.c | ||
| multifd.c | ||
| multifd.h | ||
| options.c | ||
| options.h | ||
| page_cache.c | ||
| page_cache.h | ||
| postcopy-ram.c | ||
| postcopy-ram.h | ||
| qemu-file.c | ||
| qemu-file.h | ||
| ram-compress.c | ||
| ram-compress.h | ||
| ram.c | ||
| ram.h | ||
| rdma.c | ||
| rdma.h | ||
| savevm.c | ||
| savevm.h | ||
| socket.c | ||
| socket.h | ||
| target.c | ||
| threadinfo.c | ||
| threadinfo.h | ||
| tls.c | ||
| tls.h | ||
| trace-events | ||
| trace.h | ||
| vmstate-types.c | ||
| vmstate.c | ||
| xbzrle.c | ||
| xbzrle.h | ||
| yank_functions.c | ||
| yank_functions.h | ||