Re: [PATCH] eventpoll: compute timer slack lazily in ep_poll()

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Jul 07, 2026 at 12:02:38PM -0700, Usama Arif wrote:
> ep_poll() computes the timer slack via select_estimate_accuracy() up front,
> before checking whether events are already available.
> select_estimate_accuracy() reads the clock (ktime_get_ts64()), and the
> resulting slack is only consumed by the schedule_hrtimeout_range() call on
> the blocking path.
> 
> A busy poller such as an L7 proxy event loop calls epoll_wait() at a very
> high rate and often finds events already pending, returning via
> ep_try_send_events() without ever blocking.  In that case the up-front
> slack estimation - including its clock read - is pure overhead.  read_tsc()
> attributable to select_estimate_accuracy() sometimes shows up in perf profiles
> of such a workload via the epoll_wait() path.
> 
> Move the slack estimation to the point where the thread is actually about
> to sleep.  The timeout passed to ep_poll() is already an absolute deadline
> (ep_timeout_to_timespec()), so deferring the estimate does not change the
> wakeup time; taken closer to the sleep it is, if anything, marginally more
> accurate.  On the common non-blocking path the clock read is skipped
> entirely.
> 
> Measured on a host running a Meta production workload with the following
> bpftrace script:
> 
>   #!/usr/bin/bpftrace
>   fentry:__x64_sys_epoll_wait,
>   fentry:__x64_sys_epoll_pwait   { @in[tid] = 1; }
>   fexit:__x64_sys_epoll_wait,
>   fexit:__x64_sys_epoll_pwait    { delete(@in, tid); }
>   fentry:select_estimate_accuracy /@in[tid]/ { @sea++; }
>   fentry:schedule_hrtimeout_range /@in[tid]/ { @shr++; }
>   interval:s:30 {
>       printf("sea=%lld shr=%lld wasted=%lld (%d%%)\n",
>              @sea, @shr, @sea - @shr, (@sea - @shr) * 100 / @sea);
>       exit();
>   }
> 
> Over a 30s window:
> 
> sea=3,587,704 shr=3,003,920 wasted=583,784 (16%)
> 
> So ~16% of ep_poll invocations of select_estimate_accuracy have no
> consumer.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>