linux-kernel - Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

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Message-ID: <20130820163312.GA17957@redhat.com>
Date:	Tue, 20 Aug 2013 18:33:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On 08/20, Peter Zijlstra wrote:
>
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -453,7 +453,8 @@ struct rq {
>  	u64 clock;
>  	u64 clock_task;
>  
> -	atomic_t nr_iowait;
> +	int nr_iowait_local;
> +	atomic_t nr_iowait_remote;

I am wondering how the extra lock(rq)/unlock(rq) in schedule() is bad
compared to atomic_dec.

IOW, what if we simply make rq->nr_iowait "int" and change schedule()
to update it? Something like below. Just curious.

As for nr_iowait_local + nr_iowait_remote, this doesn't look safe...
in theory nr_iowait_cpu() or even nr_iowait() can return a negative
number.

Oleg.


--- x/kernel/sched/core.c
+++ x/kernel/sched/core.c
@@ -2435,6 +2435,9 @@ need_resched:
 		rq->curr = next;
 		++*switch_count;
 
+		if (unlikely(prev->in_iowait))
+			rq->nr_iowait++;
+
 		context_switch(rq, prev, next); /* unlocks the rq */
 		/*
 		 * The context switch have flipped the stack from under us
@@ -2442,6 +2445,12 @@ need_resched:
 		 * this task called schedule() in the past. prev == current
 		 * is still correct, but it can be moved to another cpu/rq.
 		 */
+		if (unlikely(prev->in_iowait)) {
+			raw_spin_lock_irq(&rq->lock);
+			rq->nr_iowait--;
+			raw_spin_unlock_irq(&rq->lock);
+		}
+
 		cpu = smp_processor_id();
 		rq = cpu_rq(cpu);
 	} else
@@ -3939,31 +3948,24 @@ EXPORT_SYMBOL_GPL(yield_to);
  */
 void __sched io_schedule(void)
 {
-	struct rq *rq = raw_rq();
-
 	delayacct_blkio_start();
-	atomic_inc(&rq->nr_iowait);
 	blk_flush_plug(current);
 	current->in_iowait = 1;
 	schedule();
 	current->in_iowait = 0;
-	atomic_dec(&rq->nr_iowait);
 	delayacct_blkio_end();
 }
 EXPORT_SYMBOL(io_schedule);
 
 long __sched io_schedule_timeout(long timeout)
 {
-	struct rq *rq = raw_rq();
 	long ret;
 
 	delayacct_blkio_start();
-	atomic_inc(&rq->nr_iowait);
 	blk_flush_plug(current);
 	current->in_iowait = 1;
 	ret = schedule_timeout(timeout);
 	current->in_iowait = 0;
-	atomic_dec(&rq->nr_iowait);
 	delayacct_blkio_end();
 	return ret;
 }

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