Re: [PATCH 5/5] aio-posix: Separate AioPolledEvent per AioHandler

[Stefan Hajnoczi]

On Mon, Mar 10, 2025 at 12:11:44PM +0100, Kevin Wolf wrote:
> Am 10.03.2025 um 11:55 hat Stefan Hajnoczi geschrieben:
> > On Fri, Mar 07, 2025 at 11:16:34PM +0100, Kevin Wolf wrote:
> > > Adaptive polling has a big problem: It doesn't consider that an event
> > > loop can wait for many different events that may have very different
> > > typical latencies.
> > > 
> > > For example, think of a guest that tends to send a new I/O request soon
> > > after the previous I/O request completes, but the storage on the host is
> > > rather slow. In this case, getting the new request from guest quickly
> > > means that polling is enabled, but the next thing is performing the I/O
> > > request on the backend, which is slow and disables polling again for the
> > > next guest request. This means that in such a scenario, polling could
> > > help for every other event, but is only ever enabled when it can't
> > > succeed.
> > > 
> > > In order to fix this, keep a separate AioPolledEvent for each
> > > AioHandler. We will then know that the backend file descriptor always
> > > has a high latency and isn't worth polling for, but we also know that
> > > the guest is always fast and we should poll for it. This solves at least
> > > half of the problem, we can now keep polling for those cases where it
> > > makes sense and get the improved performance from it.
> > > 
> > > Since the event loop doesn't know which event will be next, we still do
> > > some unnecessary polling while we're waiting for the slow disk. I made
> > > some attempts to be more clever than just randomly growing and shrinking
> > > the polling time, and even to let callers be explicit about when they
> > > expect a new event, but so far this hasn't resulted in improved
> > > performance or even caused performance regressions. For now, let's just
> > > fix the part that is easy enough to fix, we can revisit the rest later.
> > > 
> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > ---
> > >  include/block/aio.h |  1 -
> > >  util/aio-posix.h    |  1 +
> > >  util/aio-posix.c    | 24 +++++++++++++++++++++---
> > >  util/async.c        |  2 --
> > >  4 files changed, 22 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/include/block/aio.h b/include/block/aio.h
> > > index 49f46e01cb..0ef7ce48e3 100644
> > > --- a/include/block/aio.h
> > > +++ b/include/block/aio.h
> > > @@ -233,7 +233,6 @@ struct AioContext {
> > >      int poll_disable_cnt;
> > >  
> > >      /* Polling mode parameters */
> > > -    AioPolledEvent poll;
> > >      int64_t poll_max_ns;    /* maximum polling time in nanoseconds */
> > >      int64_t poll_grow;      /* polling time growth factor */
> > >      int64_t poll_shrink;    /* polling time shrink factor */
> > > diff --git a/util/aio-posix.h b/util/aio-posix.h
> > > index 4264c518be..82a0201ea4 100644
> > > --- a/util/aio-posix.h
> > > +++ b/util/aio-posix.h
> > > @@ -38,6 +38,7 @@ struct AioHandler {
> > >  #endif
> > >      int64_t poll_idle_timeout; /* when to stop userspace polling */
> > >      bool poll_ready; /* has polling detected an event? */
> > > +    AioPolledEvent poll;
> > >  };
> > >  
> > >  /* Add a handler to a ready list */
> > > diff --git a/util/aio-posix.c b/util/aio-posix.c
> > > index 259827c7ad..2251871c61 100644
> > > --- a/util/aio-posix.c
> > > +++ b/util/aio-posix.c
> > > @@ -579,13 +579,19 @@ static bool run_poll_handlers(AioContext *ctx, 
> > > AioHandlerList *ready_list,
> > >  static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
> > >                            int64_t *timeout)
> > >  {
> > > +    AioHandler *node;
> > >      int64_t max_ns;
> > >  
> > >      if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
> > >          return false;
> > >      }
> > >  
> > > -    max_ns = qemu_soonest_timeout(*timeout, ctx->poll.ns);
> > > +    max_ns = 0;
> > > +    QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> > > +        max_ns = MAX(max_ns, node->poll.ns);
> > > +    }
> > > +    max_ns = qemu_soonest_timeout(*timeout, max_ns);
> > > +
> > >      if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
> > >          /*
> > >           * Enable poll mode. It pairs with the poll_set_started() in
> > > @@ -721,8 +727,14 @@ bool aio_poll(AioContext *ctx, bool blocking)
> > >  
> > >      /* Adjust polling time */
> > >      if (ctx->poll_max_ns) {
> > > +        AioHandler *node;
> > >          int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - 
> > > start;
> > > -        adjust_polling_time(ctx, &ctx->poll, block_ns);
> > > +
> > > +        QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
> > > +            if (QLIST_IS_INSERTED(node, node_ready)) {
> > > +                adjust_polling_time(ctx, &node->poll, block_ns);
> > > +            }
> > > +        }
> > >      }
> > >  
> > >      progress |= aio_bh_poll(ctx);
> > > @@ -772,10 +784,16 @@ void aio_context_use_g_source(AioContext *ctx)
> > >  void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
> > >                                   int64_t grow, int64_t shrink, Error 
> > > **errp)
> > >  {
> > > +    AioHandler *node;
> > > +
> > >      /* No thread synchronization here, it doesn't matter if an incorrect 
> > > value
> > >       * is used once.
> > >       */
> > 
> > If you respin this series:
> > 
> > This comment is confusing now that qemu_lockcnt_inc() is being used.
> > Lockcnt tells other threads in aio_set_fd_handler() not to remove nodes
> > from the aio_handlers list (because we're traversing the list).
> > 
> > The comment is about the poll state though, not about the aio_handlers
> > list. Moving it down to where poll_max_ns, etc are assigned would make
> > it clearer.
> 
> Yes, I can do that while applying the series.
> 
> Should I add your R-b after making the change?

Yes, please.

Stefan

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