Messages in this thread | | | Date | Sat, 7 Apr 2012 01:29:59 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns |
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On Thu, 5 Apr 2012, Prarit Bhargava wrote:
> > > > 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
You are issuing a command which puts the kernel into a state where is dumps data for several seconds with interrupts disabled. And you expect that everything can cope with that?
> If I hack in (sorry for the cut-and-paste) > .... > + 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); > then I don't see unstable messages.
That does not make your approach more correct. The HPET wraparound time is ~3 seconds, so you screwed everything already, when your dump lasts longer than that. And there are clocksources which wrap way faster.
No, you can't fix that by hacking the timer code. A wraparound CANNOT be fixed by hacks.
So instead of fiddling in the victims, please fix the root cause, i.e. that stupid sysrq-t code which should not need to have interrupts disabled just to dump all that state. If that's not possible, send a patch to the sysrq documentation and warn about the consequences.
But stay away from code which is correct already. You CANNOT fix a problem which is caused by abnormal system state by hacking the code which is exposing the problem.
All you do is making hot pathes more expensive with a very dubious value. The time related calls are hotpath functions and optimized.
Aside of that you are breaking all architectures which do not have a native 64/32 instruction.
This mult_frac stuff is not going to happen, period.
Thanks,
tglx
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