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

[Waiman Long]

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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