linux-kernel - Re: [PATCH v4 2/5] sched_clock: Optimize cache line usage

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Date:	Mon, 9 Feb 2015 01:28:01 +0000
From:	Will Deacon <will.deacon@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [PATCH v4 2/5] sched_clock: Optimize cache line usage

On Sun, Feb 08, 2015 at 12:02:37PM +0000, Daniel Thompson wrote:
> Currently sched_clock(), a very hot code path, is not optimized to
> minimise its cache profile. In particular:
> 
>   1. cd is not ____cacheline_aligned,
> 
>   2. struct clock_data does not distinguish between hotpath and
>      coldpath data, reducing locality of reference in the hotpath,
> 
>   3. Some hotpath data is missing from struct clock_data and is marked
>      __read_mostly (which more or less guarantees it will not share a
>      cache line with cd).
> 
> This patch corrects these problems by extracting all hotpath data
> into a separate structure and using ____cacheline_aligned to ensure
> the hotpath uses a single (64 byte) cache line.

Have you got any performance figures for this change, or is this just a
theoretical optimisation? It would be interesting to see what effect this
has on systems with 32-byte cachelines and also scenarios where there's
contention on the sequence counter.

Will
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