Messages in this thread | | | Date | Mon, 21 Apr 2014 15:41:54 +0200 | From | Oleg Nesterov <> | Subject | [PATCH 1/4 v2] nmi: Provide the option to issue an NMI back trace to every cpu but current |
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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.
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.
> 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.
And I think it would be better to not discuss this off-list, I added lkml.
Oleg.
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