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    SubjectRe: [PATCH] stopmachine: add stopmachine_timeout
    On Mon, Jul 14, 2008 at 11:56:18AM -0700, Jeremy Fitzhardinge wrote:
    > Rusty Russell wrote:
    > > On Monday 14 July 2008 21:51:25 Christian Borntraeger wrote:
    > >> Am Montag, 14. Juli 2008 schrieb Hidetoshi Seto:
    > >>
    > >>> + /* Wait all others come to life */
    > >>> + while (cpus_weight(prepared_cpus) != num_online_cpus() - 1) {
    > >>> + if (time_is_before_jiffies(limit))
    > >>> + goto timeout;
    > >>> + cpu_relax();
    > >>> + }
    > >>> +
    > >>>
    > >> Hmm. I think this could become interesting on virtual machines. The
    > >> hypervisor might be to busy to schedule a specific cpu at certain load
    > >> scenarios. This would cause a failure even if the cpu is not really locked
    > >> up. We had similar problems with the soft lockup daemon on s390.
    > > 5 seconds is a fairly long time. If all else fails we could have a config
    > > option to simply disable this code.

    Hmm.. probably a stupid question: but what could happen that a real cpu
    (not virtual) becomes unresponsive so that it won't schedule a MAX_RT_PRIO-1
    prioritized task for 5 seconds?

    > >> It would be good to not-use wall-clock time, but really used cpu time
    > >> instead. Unfortunately I have no idea, if that is possible in a generic
    > >> way. Heiko, any ideas?
    > >
    > > Ah, cpu time comes up again. Perhaps we should actually dig that up again;
    > > Zach and Jeremy CC'd.
    > Hm, yeah. But in this case, it's tricky. CPU time is an inherently
    > per-cpu quantity. If cpu A is waiting for cpu B, and wants to do the
    > timeout in cpu-seconds, then it has to be in *B*s cpu-seconds (and if A
    > is waiting on B,C,D,E,F... it needs to measure separate timeouts with
    > separate timebases for each other CPU). It also means that if B is
    > unresponsive but also not consuming any time (blocked in IO,
    > administratively paused, etc), then the timeout will never trigger.
    > So I think monotonic wallclock time actually makes the most sense here.

    This is asking for trouble... a config option to disable this would be
    nice. But as I don't know which problem this patch originally addresses
    it might be that this is needed anyway. So lets see why we need it first.

    > The other issue is whether cpu_relax() is the right thing to put in the
    > busywait. We don't hook it in pvops, so it's just an x86 "pause"
    > instruction, so from the hypervisor's perspective it just looks like a
    > spinning CPU. We could either hook cpu_relax() into a hypervisor yield,
    > or come up with a heavier-weight cpu_snooze() (cpu_relax() is often used
    > in loops which are expected to have a short duration, where doing a
    > hypercall+yield would be overkill).

    cpu_relax() translates to a hypervisor yield on s390. Probably makes sense
    if other architectures would do the same.

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      Last update: 2008-07-14 23:25    [W:0.023 / U:36.104 seconds]
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