linux-kernel - Re: Linux 3.1-rc9

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Message-ID: <1318879396.4172.92.camel@twins>
Date:	Mon, 17 Oct 2011 21:23:16 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Simon Kirby <sim@...tway.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Linux 3.1-rc9

On Mon, 2011-10-17 at 11:31 -0700, Linus Torvalds wrote:
> On Mon, Oct 17, 2011 at 10:54 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > I could of course propose this... but I really won't since I'm half
> > retching by now.. ;-)
> 
> Wow. Is this "ugly and fragile code week" and I just didn't get the memo?

Do I get a price?

> I do wonder if we might not fix the problem by just taking the
> *existing* lock in the right order?
> 
> IOW, how nasty would be it be to make "scheduler_tick()" just get the
> cputimer->lock outside or rq->lock?
> 
> Sure, we'd hold that lock *much* longer than we need, but how much do
> we care? Is that a lock that gets contention? It migth be the simple
> solution for now - I *would* like to get 3.1 out..

Ah, sadly the tick isn't the only one with the inverted callchain,
pretty much every callchain in the scheduler ends up in update_curr()
one way or another.

The easier way around might be something like this... even when two
threads in a process race to enable this clock the the wasted time is
pretty much of the same order as we would otherwise have wasted spinning
on the lock and the update_gt_cputime() think would end up moving the
clock fwd to the latest outcome any which way.

Humm,. Thomas anything?


---
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index c8008dd..640ded8 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -274,9 +274,7 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 	struct task_cputime sum;
 	unsigned long flags;
 
-	spin_lock_irqsave(&cputimer->lock, flags);
 	if (!cputimer->running) {
-		cputimer->running = 1;
 		/*
 		 * The POSIX timer interface allows for absolute time expiry
 		 * values through the TIMER_ABSTIME flag, therefore we have
@@ -284,8 +282,11 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 		 * it.
 		 */
 		thread_group_cputime(tsk, &sum);
+		spin_lock_irqsave(&cputimer->lock, flags);
+		cputimer->running = 1;
 		update_gt_cputime(&cputimer->cputime, &sum);
-	}
+	} else
+		spin_lock_irqsave(&cputimer->lock, flags);
 	*times = cputimer->cputime;
 	spin_unlock_irqrestore(&cputimer->lock, flags);
 }

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