Messages in this thread | | | Date | Mon, 9 Nov 2020 09:00:38 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH rfc] workqueue: honour cond_resched() more effectively. |
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On Mon, Nov 09, 2020 at 01:54:59PM +1100, NeilBrown wrote: > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 4418f5cb8324..728870965df1 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1784,7 +1784,12 @@ static inline int test_tsk_need_resched(struct task_struct *tsk) > #ifndef CONFIG_PREEMPTION > extern int _cond_resched(void); > #else > -static inline int _cond_resched(void) { return 0; } > +static inline int _cond_resched(void) > +{ > + if (current->flags & PF_WQ_WORKER) > + workqueue_cond_resched(); > + return 0; > +} > #endif > > #define cond_resched() ({ \
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 9a2fbf98fd6f..5b2e38567a0c 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5620,6 +5620,8 @@ SYSCALL_DEFINE0(sched_yield) > #ifndef CONFIG_PREEMPTION > int __sched _cond_resched(void) > { > + if (current->flags & PF_WQ_WORKER) > + workqueue_cond_resched(); > if (should_resched(0)) { > preempt_schedule_common(); > return 1;
Much hate for this.. :/ cond_resched() should be a NOP on !PREEMPT and you wreck that. Also, you call into that workqueue_cond_resched() unconditionally, even when it wouldn't have rescheduled, which seems very wrong too.
On top of all that, you're adding an extra load to the funcion :/
At some poine Paul tried to frob cond_resched() for RCU and ran into all sorts of performance issues, I'm thinking this will too.
Going by your justification for all this:
> I think that once a worker calls cond_resched(), it should be treated as > though it was run from a WQ_CPU_INTENSIVE queue, because only cpu-intensive > tasks need to call cond_resched(). This would allow other workers to be > scheduled.
I'm thinking the real problem is that you're abusing workqueues. Just don't stuff so much work into it that this becomes a problem. Or rather, if you do, don't lie to it about it.
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