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Subject: Re: [PATCH] block: fix a crash in do_task_dead()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Qian Cai <cai@lca.pw>, akpm@linux-foundation.org, hch@lst.de,
        oleg@redhat.com, gkohli@codeaurora.org, mingo@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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 <20190603123705.GB3419@hirez.programming.kicks-ass.net>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <9a75cc4f-bd14-1d98-6653-b49a2842dd16@kernel.dk>
Date:   Mon, 3 Jun 2019 10:23:09 -0600
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On 6/3/19 6:37 AM, Peter Zijlstra wrote:
> On Fri, May 31, 2019 at 03:12:13PM -0600, Jens Axboe wrote:
>> On 5/30/19 2:03 AM, Peter Zijlstra wrote:
> 
>>> What is the purpose of that patch ?! The Changelog doesn't mention any
>>> benefit or performance gain. So why not revert that?
>>
>> Yeah that is actually pretty weak. There are substantial performance
>> gains for small IOs using this trick, the changelog should have
>> included those. I guess that was left on the list...
> 
> OK. I've looked at the try_to_wake_up() path for these exact
> conditions and we're certainly sub-optimal there, and I think we can put
> much of this special case in there. Please see below.
> 
>> I know it's not super kosher, your patch, but I don't think it's that
>> bad hidden in a generic helper.
> 
> How about the thing that Oleg proposed? That is, not set a waiter when
> we know the loop is polling? That would avoid the need for this
> alltogether, it would also avoid any set_current_state() on the wait
> side of things.
> 
> Anyway, Oleg, do you see anything blatantly buggered with this patch?
> 
> (the stats were already dodgy for rq-stats, this patch makes them dodgy
> for task-stats too)
> 
> ---
>   kernel/sched/core.c | 38 ++++++++++++++++++++++++++++++++------
>   1 file changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 102dfcf0a29a..474aa4c8e9d2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1990,6 +1990,28 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   	unsigned long flags;
>   	int cpu, success = 0;
>   
> +	if (p == current) {
> +		/*
> +		 * We're waking current, this means 'p->on_rq' and 'task_cpu(p)
> +		 * == smp_processor_id()'. Together this means we can special
> +		 * case the whole 'p->on_rq && ttwu_remote()' case below
> +		 * without taking any locks.
> +		 *
> +		 * In particular:
> +		 *  - we rely on Program-Order guarantees for all the ordering,
> +		 *  - we're serialized against set_special_state() by virtue of
> +		 *    it disabling IRQs (this allows not taking ->pi_lock).
> +		 */
> +		if (!(p->state & state))
> +			goto out;
> +
> +		success = 1;
> +		trace_sched_waking(p);
> +		p->state = TASK_RUNNING;
> +		trace_sched_woken(p);
> +		goto out;
> +	}
> +
>   	/*
>   	 * If we are going to wake up a thread waiting for CONDITION we
>   	 * need to ensure that CONDITION=1 done by the caller can not be
> @@ -1999,7 +2021,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   	raw_spin_lock_irqsave(&p->pi_lock, flags);
>   	smp_mb__after_spinlock();
>   	if (!(p->state & state))
> -		goto out;
> +		goto unlock;
>   
>   	trace_sched_waking(p);
>   
> @@ -2029,7 +2051,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   	 */
>   	smp_rmb();
>   	if (p->on_rq && ttwu_remote(p, wake_flags))
> -		goto stat;
> +		goto unlock;
>   
>   #ifdef CONFIG_SMP
>   	/*
> @@ -2089,12 +2111,16 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   #endif /* CONFIG_SMP */
>   
>   	ttwu_queue(p, cpu, wake_flags);
> -stat:
> -	ttwu_stat(p, cpu, wake_flags);
> -out:
> +unlock:
>   	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
>   
> -	return success;
> +out:
> +	if (success) {
> +		ttwu_stat(p, cpu, wake_flags);
> +		return true;
> +	}
> +
> +	return false;
>   }
>   
>   /**

Let me run some tests with this vs mainline vs blk wakeup hack removed.


-- 
Jens Axboe