linux-kernel - Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers and writers of load

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Date:	Mon, 30 Nov 2015 23:29:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Yuyang Du <yuyang.du@...el.com>
Subject: Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers
 and writers of load_avg

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().
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