linux-kernel - Re: [PATCH] clocksource, prevent overflow in clocksource

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Date:	Thu, 05 Apr 2012 08:27:13 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	John Stultz <johnstul@...ibm.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Salman Qazi <sqazi@...gle.com>, stable@...nel.org
Subject: Re: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns

> 
> So what kernel version are you using?

I retested using top of the linux.git tree, running

echo 1 > /proc/sys/kernel/sysrq
for i in `seq 10000`; do sleep 1000 & done
echo t > /proc/sysrq-trigger

and I no longer see a problem.  However, if I increase the number of threads to
1000/cpu I get

Clocksource %s unstable (delta = -429565427)
Clocksource switching to hpet

> to narrow down if you're  problem  is currently present in mainline or only in
> older kernels, as that will help us find the proper fix.

If I hack in (sorry for the cut-and-paste)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index c958338..f38b8d0 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -279,11 +279,16 @@ static void clocksource_watchdog(unsigned long data)
                        continue;
                }

-               wd_nsec = clocksource_cyc2ns((wdnow - cs->wd_last) & watchdog->m
-                                            watchdog->mult, watchdog->shift);
+               /*wd_nsec = clocksource_cyc2ns((wdnow - cs->wd_last) & watchdog-
+                                            watchdog->mult, watchdog->shift);*/
+               wd_nsec = mult_frac(((wdnow - cs->wd_last), watchdog->mult,
+                                   1UL << watchdog->shift);
+
+               /*cs_nsec = clocksource_cyc2ns((csnow - cs->cs_last) &
+                                            cs->mask, cs->mult, cs->shift);*/
+               cs_nsec = mult_frac(((csnow - cs->cs_last), cs->mult,
+                                   1UL << cs->shift);

-               cs_nsec = clocksource_cyc2ns((csnow - cs->cs_last) &
-                                            cs->mask, cs->mult, cs->shift);
                cs->cs_last = csnow;
                cs->wd_last = wdnow;


then I don't see unstable messages.

I think the problem is still here but it only happens in extreme cases.

P.
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