mm: Page allocation from buddy system might delay the tasks on different SMP cores

mm: Page allocation from buddy system might delay the tasks on different SMP cores

[GeHao Kang]

Hi Mel,
Through my experiment in SMP system, I find the mm_page_alloc_zone_locked
event produced by the fork system call on one CPU might delay the task
on another CPU.  According to the events-keme.txt in kernel documents,
the interrupts are disabled and cache lines between CPUs are dirtied
when this event happens.  Therefore, I am afraid that a task might be
interfered by the tasks on different CPUs
which at the same time request memory from the buddy allocator.

My questions are as follows:
    * Is it necessary to disable interrupts when allocating/freeing memory from
      buddy system?
    * Why the cache lines between CPUs are dirtied by the allocation in
      buddy system?

Thanks,
- Kang

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Re: mm: Page allocation from buddy system might delay the tasks on different SMP cores

[Mel Gorman]

On Wed, Apr 20, 2016 at 12:07:21AM +0800, GeHao Kang wrote:
> Hi Mel,
> Through my experiment in SMP system, I find the mm_page_alloc_zone_locked
> event produced by the fork system call on one CPU might delay the task
> on another CPU.  According to the events-keme.txt in kernel documents,
> the interrupts are disabled and cache lines between CPUs are dirtied
> when this event happens.  Therefore, I am afraid that a task might be
> interfered by the tasks on different CPUs
> which at the same time request memory from the buddy allocator.
> 
> My questions are as follows:
>     * Is it necessary to disable interrupts when allocating/freeing memory from
>       buddy system?

Yes.

>     * Why the cache lines between CPUs are dirtied by the allocation in
>       buddy system?
> 

Because a page can be freed on one CPU and allocated using another.

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