Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

[Shrikanth Hegde]

On 10/22/24 00:51, Ankur Arora wrote:
> Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> writes:
>
> > On 10/9/24 22:24, Ankur Arora wrote:
> > > resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
> > > without rescheduling for more than the latency_warn_ms period.
> >
> > I am bit confused here. Why do we need to warn if LAZY is set for a long time?
> >
> > If lazy set, the subsequent tick, it would be set to upgraded to NEED_RESCHED.
> >
> > Since the value of latency_warn_ms=100ms, that means even on system with HZ=100,
> > that means 10 ticks before that warning would be printed no?
>
> That's a fair point. However, the assumption there is that there are no
> bugs in upgrade on tick or that there's no situation in which the tick
> is off for a prolonged period.

ok.

But if tick is off, then ticks_without_resched isn't incremented either. 
IIUC, this check is for situation when NR is set and tick is on.

> Ankur
>
> > IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of pending
> > need_resched") has the concern of need_resched set but if it is non-preemptible
> > kernel it would spend a lot of time in kernel mode. In that case print a
> > warning.
> >
> > If someone enables Lazy, that means it is preemptible and probably this whole
> > notion of resched_latency_warn doesn't apply to lazy. Please correct me if i am
> > not understanding this correctly.
> >
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> > > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> > > ---
> > >    kernel/sched/core.c  | 2 +-
> > >    kernel/sched/debug.c | 7 +++++--
> > >    2 files changed, 6 insertions(+), 3 deletions(-)
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 694bfcf153cb..1229766b704e 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
> > >    	if (sysctl_resched_latency_warn_once && warned_once)
> > >    		return 0;
> > >    -	if (!need_resched() || !latency_warn_ms)
> > > +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
> > >    		return 0;
> > >      	if (system_state == SYSTEM_BOOTING)
> > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > > index 9abcc6ead11b..f0d551ba64bb 100644
> > > --- a/kernel/sched/debug.c
> > > +++ b/kernel/sched/debug.c
> > > @@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p)
> > >    void resched_latency_warn(int cpu, u64 latency)
> > >    {
> > >    	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
> > > +	char *nr;
> > > +
> > > +	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
> > >      	WARN(__ratelimit(&latency_check_ratelimit),
> > > -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
> > > +	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
> > >    	     "without schedule\n",
> > > -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
> > > +	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
> > >    }
>
>
> --
> ankur