On Wed, Mar 17, 2021 at 1:31 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Josh Don <joshdon@xxxxxxxxxx> wrote:
>
> > +static inline u64 resched_latency_check(struct rq *rq)
> > +{
> > + int latency_warn_ms = READ_ONCE(sysctl_resched_latency_warn_ms);
> > + bool warn_only_once = (latency_warn_ms == RESCHED_DEFAULT_WARN_LATENCY_MS);
> > + u64 need_resched_latency, now = rq_clock(rq);
> > + static bool warned_once;
> > +
> > + if (warn_only_once && warned_once)
> > + return 0;
> > +
> > + if (!need_resched() || latency_warn_ms < 2)
> > + return 0;
> > +
> > + /* Disable this warning for the first few mins after boot */
> > + if (now < RESCHED_BOOT_QUIET_SEC * NSEC_PER_SEC)
> > + return 0;
> > +
> > + if (!rq->last_seen_need_resched_ns) {
> > + rq->last_seen_need_resched_ns = now;
> > + rq->ticks_without_resched = 0;
> > + return 0;
> > + }
> > +
> > + rq->ticks_without_resched++;
>
> So AFAICS this will only really do something useful on full-nohz
> kernels with sufficiently long scheduler ticks, right?
Not quite sure what you mean; it is actually the inverse? Since we
rely on the tick to detect the resched latency, on nohz-full we won't
have detection on cpus running a single thread. The ideal scenario is
!nohz-full and tick interval << warn_ms.
> On other kernels the scheduler tick interrupt, when it returns to
> user-space, will trigger a reschedule if it sees a need_resched.
True for the case where we return to userspace, but we could instead
be executing in a non-preemptible region of the kernel. This is where
we've seen/fixed kernel bugs.
Best,
Josh
