On Mon, Apr 21, 2014 at 03:41:54PM +0200, Oleg Nesterov wrote:
> On 04/21, Don Zickus wrote:
> >
> > On Tue, Apr 15, 2014 at 07:26:49PM +0200, Oleg Nesterov wrote:
> > > On 04/15, Oleg Nesterov wrote:
> > > >
> > > > Looking at https://lkml.org/lkml/2014/4/4/469... It seems that 2/4 can
> > > > be
> > > > simplified, you can simply remove smp_processor_id() from backtrace_mask
> > > > if !include_self and use apic->send_IPI_mask(backtrace_mask). But this
> > > > is
> > > > minor, I won't insist.
> > >
> > > And in fact, I do not understand why arch_trigger_all_cpu_backtrace()
> > > doesn't
> > > disable preemption. OK, probably we can simply ignore the race with cpu
> > > hotplug.
> > >
> > > But it seems that your patch makes the things worse. Lets look at, say,
> > > numachip_send_IPI_mask_allbutself(). The usage of smp_processor_id() is
> > > obviously racy but perhaps we do not care again. But we do not want a
> > > warning
> > > from debug_smp_processor_id().
> >
> > Good point. I forgot that going from all cpus down to allbutself,
> > preemption now matters.
>
> I am not sure it actually matters wrt "show other CPU's traces". If the
> preemption
> is possible then the caller can be preempted even before it sends ipi.
>
> OTOH I think it does matter anyway, even without your patch, otherwise the
> usage
> of cpu_online_mask is racy and we can hit the "Wait for up to 10 seconds"
> case.
Hmm, I understand what you are saying now.
>
> Btw...
>
> /* Wait for up to 10 seconds for all CPUs to do the backtrace */
> for (i = 0; i < 10 * 1000; i++) {
> if (cpumask_empty(to_cpumask(backtrace_mask)))
> break;
> mdelay(1);
> }
>
> OK, but perhaps we should clear backtrace_mask if we return due to timeout.
I can look at that.
>
> > does disabling preemption help in the cpu
> > hotplug case?
>
> Yes. But I'd suggest to change your patch to use get_cpu() instead of
> preempt_disable/smp_processor_id.
ok. Originally I was thinking of the remote hotplug cpu case, which
pre-emption won't block. But forgot about the local cpu hotplug case.
>
> And I think it would be better to not discuss this off-list, I added lkml.
>
> Oleg.
>
Thanks!
Cheers,
Don
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