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Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers and writers of load_avg

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On 11/30/2015 05:29 PM, Peter Zijlstra wrote:
> On Mon, Nov 30, 2015 at 02:13:32PM -0500, Waiman Long wrote:
>>> This would only work if the structure itself is allocated with cacheline
>>> alignment, and looking at sched_create_group(), we use a plain kzalloc()
>>> for this, which doesn't guarantee any sort of alignment beyond machine
>>> word size IIRC.
>> With a RHEL 6 derived .config file, the size of the task_group structure was
>> 460 bytes on a 32-bit x86 kernel. Adding a ____cacheline_aligned tag
>> increase the size to 512 bytes. So it did make the structure a multiple of
>> the cacheline size. With both slub and slab, the allocated task group
>> pointers from kzalloc() in sched_create_group() were all multiples of 0x200.
>> So they were properly aligned for the ____cacheline_aligned tag to work.
> Not sure we should rely on sl*b doing the right thing here.
> KMALLOC_MIN_ALIGN is explicitly set to sizeof(long long). If you want
> explicit alignment, one should use KMEM_CACHE().

I think the current kernel use power-of-2 kmemcaches to satisfy kalloc() 
requests except when the size is less than or equal to 192 where there 
are some non-power-of-2 kmemcaches available. Given that the task_group 
structure is large enough with FAIR_GROUP_SCHED enabled, we shouldn't 
hit the case that the allocated buffer is not cacheline aligned.

Cheers,
Longman
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Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Waiman Long <waiman.long@hpe.com> - 2015-11-30 20:20 +0100
  Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Peter Zijlstra <peterz@infradead.org> - 2015-11-30 23:20 +0100
    Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Waiman Long <waiman.long@hpe.com> - 2015-12-01 05:00 +0100
      Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Peter Zijlstra <peterz@infradead.org> - 2015-12-01 09:50 +0100
        Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for  readers and writers of load_avg Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-12-01 11:50 +0100
        Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Waiman Long <waiman.long@hpe.com> - 2015-12-02 19:50 +0100
  Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Peter Zijlstra <peterz@infradead.org> - 2015-11-30 23:30 +0100
    Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Waiman Long <waiman.long@hpe.com> - 2015-12-01 05:10 +0100
      Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Peter Zijlstra <peterz@infradead.org> - 2015-12-01 09:50 +0100
        Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Waiman Long <waiman.long@hpe.com> - 2015-12-02 19:50 +0100
  Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers  and writers of load_avg Peter Zijlstra <peterz@infradead.org> - 2015-11-30 23:40 +0100

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