Re: [PATCH v3] util/async: make bh_aio_poll() O(1)

[Stefan Hajnoczi]

On Fri, Feb 21, 2020 at 9:40 AM Stefan Hajnoczi <address@hidden> wrote:
>
> The ctx->first_bh list contains all created BHs, including those that
> are not scheduled.  The list is iterated by the event loop and therefore
> has O(n) time complexity with respected to the number of created BHs.
>
> Rewrite BHs so that only scheduled or deleted BHs are enqueued.
> Only BHs that actually require action will be iterated.
>
> One semantic change is required: qemu_bh_delete() enqueues the BH and
> therefore invokes aio_notify().  The
> tests/test-aio.c:test_source_bh_delete_from_cb() test case assumed that
> g_main_context_iteration(NULL, false) returns false after
> qemu_bh_delete() but it now returns true for one iteration.  Fix up the
> test case.
>
> This patch makes aio_compute_timeout() and aio_bh_poll() drop from a CPU
> profile reported by perf-top(1).  Previously they combined to 9% CPU
> utilization when AioContext polling is commented out and the guest has 2
> virtio-blk,num-queues=1 and 99 virtio-blk,num-queues=32 devices.
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> v3:
>  * Use QSLIST_FOREACH_RCU() and QSLIST_FIRST_RCU() [Paolo]

I forgot to include Paolo's R-b that he gave conditional on making this change:

Reviewed-by: Paolo Bonzini <address@hidden>