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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 11, 2025 at 03:35:35PM +0200, Kuba Piecuch wrote:
> On Mon, Aug 11, 2025 at 10:36 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > 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?
> 
> The code calling sched_yield() was in the wait loop for a spinlock. It
> would repeatedly yield until the compare-and-swap instruction succeeded
> in acquiring the lock. This code runs in the SIGPROF handler.

Well, then don't do that... userspace spinlocks are terrible, and
bashing yield like that isn't helpful either.

Throttling yield seems like entirely the wrong thing to do. Yes, yield()
is poorly defined (strictly speaking UB for anything not FIFO/RR) but
making it actively worse doesn't seem helpful.

The whole itimer thing is not scalable -- blaming that on yield seems
hardly fair.

Why not use timer_create(), with CLOCK_THREAD_CPUTIME_ID and
SIGEV_SIGNAL instead?