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![]() 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 |