linux-kernel - Re: [PATCH] softlockup: stop spurious softlockup messages due to overflow

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Message-ID: <20100318152535.GB12564@elte.hu>
Date:	Thu, 18 Mar 2010 16:25:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Colin Ian King <colin.king@...onical.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] softlockup: stop spurious softlockup messages due to
 overflow


* Colin Ian King <colin.king@...onical.com> wrote:

> On Tue, 2010-03-16 at 11:12 +0100, Ingo Molnar wrote:
> > * Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > Le lundi 15 mars 2010 ?? 14:01 +0000, Colin Ian King a ??crit :
> > > > Ensure additions on touch_ts do not overflow.  This can occur when
> > > > the top 32 bits of the TSC reach 0xffffffff causing additions to
> > > > touch_ts to overflow and this in turn generates spurious softlockup
> > > > warnings.
> > > > 
> > > > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > > > ---
> > > >  kernel/softlockup.c |    6 +++---
> > > >  1 files changed, 3 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/kernel/softlockup.c b/kernel/softlockup.c
> > > > index 0d4c789..90d9aa0 100644
> > > > --- a/kernel/softlockup.c
> > > > +++ b/kernel/softlockup.c
> > > > @@ -111,10 +111,10 @@ int proc_dosoftlockup_thresh(struct ctl_table *table, int write,
> > > >  void softlockup_tick(void)
> > > >  {
> > > >  	int this_cpu = smp_processor_id();
> > > > -	unsigned long touch_ts = per_cpu(softlockup_touch_ts, this_cpu);
> > > > +	unsigned long long touch_ts = per_cpu(softlockup_touch_ts, this_cpu);
> > > >  	unsigned long print_ts;
> > > >  	struct pt_regs *regs = get_irq_regs();
> > > > -	unsigned long now;
> > > > +	unsigned long long now;
> > > >  
> > > >  	/* Is detection switched off? */
> > > >  	if (!per_cpu(softlockup_watchdog, this_cpu) || softlockup_thresh <= 0) {
> > > > @@ -165,7 +165,7 @@ void softlockup_tick(void)
> > > >  	per_cpu(softlockup_print_ts, this_cpu) = touch_ts;
> > > >  
> > > >  	spin_lock(&print_lock);
> > > > -	printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %lus! [%s:%d]\n",
> > > > +	printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %llus! [%s:%d]\n",
> > > >  			this_cpu, now - touch_ts,
> > > >  			current->comm, task_pid_nr(current));
> > > >  	print_modules();
> > > 
> > > This looks wrong, touch_ts is a long, not a long long.
> > 
> > Could be increased to long long - but that's probably overkill as the touch_ts 
> > is in seconds, so the scope of comparisons should never truly get even close 
> > to ~2^31.
> > 
> > > You probably want to change the comparisons instead.
> > > 
> > > if (now > touch_ts + softlockup_thresh/2)
> > > 	wake_up_process(per_cpu(softlockup_watchdog, this_cpu));
> > > if (now <= (touch_ts + softlockup_thresh))
> > > 	return;
> > > 	
> > > ->
> > > 
> > > if ((long)(now - touch_ts) > softlockup_thresh/2)
> > > 	wake_up_process(per_cpu(softlockup_watchdog, this_cpu));
> > > if ((long)(now - touch_ts) <= softlockup_thresh))
> > > 	return;
> > > 
> > > Or use standard time_after()/time_before() macros.
> > 
> > Yeah, time_after/before would work better i suspect.
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> Using time_after/before:
> 
> diff --git a/kernel/softlockup.c b/kernel/softlockup.c
> index 0d4c789..4b493f6 100644
> --- a/kernel/softlockup.c
> +++ b/kernel/softlockup.c
> @@ -155,11 +155,11 @@ void softlockup_tick(void)
>          * Wake up the high-prio watchdog task twice per
>          * threshold timespan.
>          */
> -       if (now > touch_ts + softlockup_thresh/2)
> +       if (time_after(now - softlockup_thresh/2, touch_ts))
>                 wake_up_process(per_cpu(softlockup_watchdog, this_cpu));
>  
>         /* Warn about unreasonable delays: */
> -       if (now <= (touch_ts + softlockup_thresh))
> +       if (time_before_eq(now - softlockup_thresh, touch_ts))
>                 return;

Ok, that looks like the most readable variant, agreed?

	Ingo
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