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

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Date:	Wed, 02 Dec 2015 13:44:50 -0500
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 12/01/2015 03:47 AM, Peter Zijlstra wrote:
> 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.

Thanks for the tip. I have just sent out an updated patch set which 
create a cache-aligned memcache for task group. That should work under 
all kernel config setting.

Cheers,
Longman
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