Date: Tue, 1 Dec 2015 09:47:48 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>, Paul Turner <pjt@google.com>,
        Ben Segall <bsegall@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Yuyang Du <yuyang.du@intel.com>
Subject: Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers
 and writers of load_avg
Message-ID: <20151201084748.GJ3816@twins.programming.kicks-ass.net>
References: <1448478580-26467-1-git-send-email-Waiman.Long@hpe.com>
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 <20151130102240.GH17308@twins.programming.kicks-ass.net>
 <565C9FDC.9020603@hpe.com>
 <20151130222936.GF3816@twins.programming.kicks-ass.net>
 <565D1B63.3070908@hpe.com>
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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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