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

[Kevin Wolf <kwolf@redhat.com>]

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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?

Kevin

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