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aio-posix: avoid reacquiring rcu_read_lock() when polling 

The first rcu_read_lock/unlock() is expensive.  Nested calls are cheap.

This optimization increases IOPS from 73k to 162k with a Linux guest
that has 2 virtio-blk,num-queues=1 and 99 virtio-blk,num-queues=32
devices.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20200218182708.914552-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-02-18 18:27:08 +00:00
parent c9b7d9ec21
commit f25c0b5479
1 changed files with 11 additions and 0 deletions
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11
util/aio-posix.c
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@ -15,6 +15,7 @@
#include "qemu/osdep.h"
#include "block/block.h"
#include "qemu/rcu.h"
#include "qemu/rcu_queue.h"
#include "qemu/sockets.h"
#include "qemu/cutils.h"
@ -514,6 +515,16 @@ static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
bool progress = false;
AioHandler *node;
/*
* Optimization: ->io_poll() handlers often contain RCU read critical
* sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
* -> rcu_read_lock() -> ... sequences with expensive memory
* synchronization primitives. Make the entire polling loop an RCU
* critical section because nested rcu_read_lock()/rcu_read_unlock() calls
* are cheap.
*/
RCU_READ_LOCK_GUARD();
QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
if (!node->deleted && node->io_poll &&
aio_node_check(ctx, node->is_external) &&