Re: [Qemu-devel] [PATCH 2/3] aio-posix: compute timeout before polling

[Fam Zheng]

On Wed, 09/12 19:10, Paolo Bonzini wrote:
> This is a preparation for the next patch, and also a very small
> optimization.  Compute the timeout only once, before invoking
> try_poll_mode, and adjust it in run_poll_handlers.  The adjustment
> is the polling time when polling fails, or zero (non-blocking) if
> polling succeeds.
> 
> Fixes: 70232b5253a3c4e03ed1ac47ef9246a8ac66c6fa
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  util/aio-posix.c  | 59 +++++++++++++++++++++++++++--------------------
>  util/trace-events |  4 ++--
>  2 files changed, 36 insertions(+), 27 deletions(-)
> 
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index 5c29380575..21d8987179 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -490,7 +490,7 @@ static void add_pollfd(AioHandler *node)
>      npfd++;
>  }
>  
> -static bool run_poll_handlers_once(AioContext *ctx)
> +static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
>  {
>      bool progress = false;
>      AioHandler *node;
> @@ -500,6 +500,7 @@ static bool run_poll_handlers_once(AioContext *ctx)
>              aio_node_check(ctx, node->is_external) &&
>              node->io_poll(node->opaque) &&
>              node->opaque != &ctx->notifier) {
> +            *timeout = 0;
>              progress = true;
>          }
>  
> @@ -522,31 +523,38 @@ static bool run_poll_handlers_once(AioContext *ctx)
>   *
>   * Returns: true if progress was made, false otherwise
>   */
> -static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
> +static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t 
> *timeout)
>  {
>      bool progress;
> -    int64_t end_time;
> +    int64_t start_time, elapsed_time;
>  
>      assert(ctx->notify_me);
>      assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
>  
> -    trace_run_poll_handlers_begin(ctx, max_ns);
> -
> -    end_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + max_ns;
> +    trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
>  
> +    start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      do {
> -        progress = run_poll_handlers_once(ctx);
> -    } while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time
> +        progress = run_poll_handlers_once(ctx, timeout);
> +        elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
> +    } while (!progress && elapsed_time < max_ns
>               && !atomic_read(&ctx->poll_disable_cnt));
>  
> -    trace_run_poll_handlers_end(ctx, progress);
> +    /* If time has passed with no successful polling, adjust *timeout to
> +     * keep the same ending time.
> +     */
> +    if (*timeout != -1) {
> +        *timeout -= MIN(*timeout, elapsed_time);
> +    }
>  
> +    trace_run_poll_handlers_end(ctx, progress, *timeout);
>      return progress;
>  }
>  
>  /* try_poll_mode:
>   * @ctx: the AioContext
> - * @blocking: busy polling is only attempted when blocking is true
> + * @timeout: timeout for blocking wait, computed by the caller and updated if
> + *    polling succeeds.
>   *
>   * ctx->notify_me must be non-zero so this function can detect aio_notify().
>   *
> @@ -554,19 +562,16 @@ static bool run_poll_handlers(AioContext *ctx, int64_t 
> max_ns)
>   *
>   * Returns: true if progress was made, false otherwise
>   */
> -static bool try_poll_mode(AioContext *ctx, bool blocking)
> +static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
>  {
> -    if (blocking && ctx->poll_max_ns && 
> !atomic_read(&ctx->poll_disable_cnt)) {
> -        /* See qemu_soonest_timeout() uint64_t hack */
> -        int64_t max_ns = MIN((uint64_t)aio_compute_timeout(ctx),
> -                             (uint64_t)ctx->poll_ns);
> +    /* See qemu_soonest_timeout() uint64_t hack */
> +    int64_t max_ns = MIN((uint64_t)*timeout, (uint64_t)ctx->poll_ns);
>  
> -        if (max_ns) {
> -            poll_set_started(ctx, true);
> +    if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
> +        poll_set_started(ctx, true);
>  
> -            if (run_poll_handlers(ctx, max_ns)) {
> -                return true;
> -            }
> +        if (run_poll_handlers(ctx, max_ns, timeout)) {
> +            return true;
>          }
>      }
>  
> @@ -575,7 +580,7 @@ static bool try_poll_mode(AioContext *ctx, bool blocking)
>      /* Even if we don't run busy polling, try polling once in case it can 
> make
>       * progress and the caller will be able to avoid ppoll(2)/epoll_wait(2).
>       */
> -    return run_poll_handlers_once(ctx);
> +    return run_poll_handlers_once(ctx, timeout);
>  }
>  
>  bool aio_poll(AioContext *ctx, bool blocking)
> @@ -605,8 +610,14 @@ bool aio_poll(AioContext *ctx, bool blocking)
>          start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      }
>  
> -    progress = try_poll_mode(ctx, blocking);
> -    if (!progress) {
> +    timeout = blocking ? aio_compute_timeout(ctx) : 0;
> +    progress = try_poll_mode(ctx, &timeout);
> +    assert(!(timeout && progress));
> +
> +    /* If polling is allowed, non-blocking aio_poll does not need the
> +     * system call---a single round of run_poll_handlers_once suffices.
> +     */
> +    if (timeout || atomic_read(&ctx->poll_disable_cnt)) {
>          assert(npfd == 0);
>  
>          /* fill pollfds */
> @@ -620,8 +631,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
>              }
>          }
>  
> -        timeout = blocking ? aio_compute_timeout(ctx) : 0;
> -
>          /* wait until next event */
>          if (aio_epoll_check_poll(ctx, pollfds, npfd, timeout)) {
>              AioHandler epoll_handler;
> diff --git a/util/trace-events b/util/trace-events
> index 4822434c89..79569b7fdf 100644
> --- a/util/trace-events
> +++ b/util/trace-events
> @@ -1,8 +1,8 @@
>  # See docs/devel/tracing.txt for syntax documentation.
>  
>  # util/aio-posix.c
> -run_poll_handlers_begin(void *ctx, int64_t max_ns) "ctx %p max_ns %"PRId64
> -run_poll_handlers_end(void *ctx, bool progress) "ctx %p progress %d"
> +run_poll_handlers_begin(void *ctx, int64_t max_ns, int64_t timeout) "ctx %p 
> max_ns %"PRId64 " timeout %"PRId64
> +run_poll_handlers_end(void *ctx, bool progress, int64_t timeout) "ctx %p 
> progress %d new timeout %"PRId64
>  poll_shrink(void *ctx, int64_t old, int64_t new) "ctx %p old %"PRId64" new 
> %"PRId64
>  poll_grow(void *ctx, int64_t old, int64_t new) "ctx %p old %"PRId64" new 
> %"PRId64
>  
> -- 
> 2.17.1
> 
> 

Reviewed-by: Fam Zheng <address@hidden>