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

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Message-ID: <20151201084748.GJ3816@twins.programming.kicks-ass.net>
Date:	Tue, 1 Dec 2015 09:47:48 +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 11:00:35PM -0500, Waiman Long wrote:

> 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.

Using out-of-object storage is allowed (none of the existing sl*b
allocators do so iirc).

That is, its perfectly valid for a sl*b allocator for 64 byte objects to
allocate 72 bytes for each object and use the 'spare' 8 bytes for object
tracking or whatnot.

That would respect the minimum alignment guarantee of 8 bytes but not
provide the 'expected' object size alignment you're assuming.

Also, we have the proper interfaces to request the explicit alignment
for a reason. So if you need the alignment for correctness, use those.
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