Re: [PATCH] mm, page_alloc: fix the duplicate save/ressave irq

[zhong jiang <zhongjiang@huawei.com>]

On 2017/3/13 22:26, Mel Gorman wrote:
> On Mon, Mar 13, 2017 at 09:59:33PM +0800, zhong jiang wrote:
>> On 2017/3/13 19:19, Mel Gorman wrote:
>>> On Mon, Mar 13, 2017 at 04:02:54PM +0800, zhongjiang wrote:
>>>> From: zhong jiang <zhongjiang@huawei.com>
>>>>
>>>> when commit 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
>>>> introduced to the mainline, free_pcppages_bulk irq_save/resave to protect
>>>> the IRQ context. but drain_pages_zone fails to clear away the irq. because
>>>> preempt_disable have take effect. so it safely remove the code.
>>>>
>>>> Fixes: 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
>>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>> It's not really a fix but is this even measurable?
>>>
>>> The reason the IRQ saving was preserved was for callers that are removing
>>> the CPU where it's not 100% clear if the CPU is protected from IPIs at
>>> the time the pcpu drain takes place. It may be ok but the changelog
>>> should include an indication that it has been considered and is known to
>>> be fine versus CPU hotplug.
>>>
>> you mean the removing cpu maybe  handle the IRQ, it will result in the incorrect pcpu->count ?
>>
> Yes, if it hasn't had interrupts disabled yet at the time of the drain.
> I didn't check, it probably is called from a context that disables
> interrupts but the fact you're not sure makes me automatically wary of
> the patch particularly given how little difference it makes for the common
> case where direct reclaim failed triggering a drain.
  Ok, I will test the benefits or not when direct reclaim failed and trigger a drain.  
>> but I don't sure that dying cpu remain handle the IRQ.
>>
> You'd need to be certain to justify the patch.
>
 Truely , Still  undecided it i rational at least theretically.  

Thanks
zhongjiang

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