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.
