linux-kernel - Re: [PATCH] sched_clock: fix jiffie fallback clock

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Date:	Wed, 17 Sep 2008 21:10:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	David Howells <dhowells@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_clock: fix jiffie fallback clock


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> David pointed out that the default sched_clock() fallback is broken in 
> that it wraps too soon. Fix this by using the 64 bit jiffie value so 
> that we're large enough to overflow properly.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> CC: David Howells <dhowells@...hat.com>

FYI, -tip testing found bootup hard-lockups today on 32-bit 
testsystems.

I've bisected it down to this change. After some head-scratching and an 
hour of debugging, it turns out that this aspect:

> @@ -46,10 +46,8 @@ u64 native_sched_clock(void)
>        *   very important for it to be as fast as the platform
>        *   can achive it. )
>        */
> -     if (unlikely(tsc_disabled)) {
> -             /* No locking but a rare wrong value is not a big deal: */
> -             return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
> -     }
> +     if (unlikely(tsc_disabled))
> +             return (get_jiffies_64() - INITIAL_JIFFIES) * (NSEC_PER_SEC/HZ);

is the buggy one: it calls get_jiffies_64() which does:

                seq = read_seqbegin(&xtime_lock);

that's not a very wise thing to do within xtime-locked sections - and we 
do call cpu_clock()/sched_clock() in a number of xtime-locked sections.

So i've zapped this commit for the time being, this needs to be solved 
differently.

	Ingo
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