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Date:   Mon, 9 Nov 2020 09:00:38 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Tejun Heo <tj@kernel.org>, Lai Jiangshan <jiangshanlai@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH rfc] workqueue: honour cond_resched() more effectively.
Message-ID: <20201109080038.GY2594@hirez.programming.kicks-ass.net>
References: <87v9efp7cs.fsf@notabene.neil.brown.name>
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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 tas=
k_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
> =20
>  #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-intensi=
ve
> 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.