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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130820175429.GI3258@twins.programming.kicks-ass.net>
Date:	Tue, 20 Aug 2013 19:54:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Tue, Aug 20, 2013 at 06:33:12PM +0200, Oleg Nesterov wrote:
> On 08/20, Peter Zijlstra wrote:

> 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.

I'm too tired to see how its different, I'll think on it :-)


That said:

> --- 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);
> +		}

This seems like the wrong place, this is where you return from
schedule() running another task, not where the task you just send to
sleep wakes up.

If you want to to this, the nr_iowait decrement needs to be in the
wakeup path someplace.

>  		cpu = smp_processor_id();
>  		rq = cpu_rq(cpu);
>  	} else
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/