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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. 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 >> Cc: Peter Ziljstra >> Cc: Juri Lelli >> Cc: Vincent Guittot >> Signed-off-by: Ankur Arora >> --- >> 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