Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists viaIPI from page allocator context

[Gilad Ben-Yossef]

On Thu, Jan 12, 2012 at 5:08 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Thu, 2012-01-12 at 16:51 +0200, Gilad Ben-Yossef wrote:
>> What I can't figure out is why we don't need  get/put_online_cpus()
>> pair around each and every call
>> to on_each_cpu everywhere? and if we do, perhaps making it a part of
>> on_each_cpu is the way to go?
>>
>> Something like:
>>
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index f66a1b2..cfa3882 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
>> *info, int wait)
>>  {
>>         unsigned long flags;
>>
>> +       BUG_ON(in_atomic());
>> +
>> +       get_online_cpus();
>>         preempt_disable();
>
> Your preempt_disable() here serializes against hotplug..

I'm probably daft, but why didn't it work for the page allocator then?

Mel's description reads: "Part of the problem is the page allocator is
sending IPIs using
on_each_cpu() without calling get_online_cpus() to prevent changes
to the online cpumask."

on_each_cpu() disables preemption in master as well...

Gilad




Thanks,
Gilad

-- 
Gilad Ben-Yossef
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gilad@xxxxxxxxxxxxx
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"Unfortunately, cache misses are an equal opportunity pain provider."
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