Messages in this thread | | | Date | Sat, 07 Apr 2012 09:47:44 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns |
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On 04/06/2012 07:29 PM, Thomas Gleixner wrote: > 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?
Yes actually. I do expect that everything "copes" with it. I don't find it unreasonable with system sizes increasing that functionality that has been around for years works.
However, I also understand that no one expected or saw this problem -- I'm not blaming anyone or screaming "Hey! This is broken!!!".
> >> 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.
Okay -- thanks for the info. Much appreciated.
P.
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