Re: [RFC PATCH 0/3] sched: add ability to throttle sched_yield() calls to reduce contention

[Peter Zijlstra]

On Fri, Aug 08, 2025 at 08:02:47PM +0000, Kuba Piecuch wrote:
> Problem statement
> =================
> 
> Calls to sched_yield() can touch data shared with other threads.
> Because of this, userspace threads could generate high levels of contention
> by calling sched_yield() in a tight loop from multiple cores.
> 
> For example, if cputimer is enabled for a process (e.g. through
> setitimer(ITIMER_PROF, ...)), all threads of that process
> will do an atomic add on the per-process field
> p->signal->cputimer->cputime_atomic.sum_exec_runtime inside
> account_group_exec_runtime(), which is called inside update_curr().
> 
> Currently, calling sched_yield() will always call update_curr() at least
> once in schedule(), and potentially one more time in yield_task_fair().
> Thus, userspace threads can generate quite a lot of contention for the
> cacheline containing cputime_atomic.sum_exec_runtime if multiple threads of
> a process call sched_yield() in a tight loop.
> 
> At Google, we suspect that this contention led to a full machine lockup in
> at least one instance, with ~50% of CPU cycles spent in the atomic add
> inside account_group_exec_runtime() according to
> `perf record -a -e cycles`.

I've gotta ask, WTH is your userspace calling yield() so much?