linux-kernel - Re: [PATCH v4 0/8] Add hardware prefetch control driver for A64FX and x86

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Message-ID: <57184edb-8a04-b4bd-969b-bca152375d86@marcan.st>
Date:   Thu, 19 May 2022 17:29:39 +0900
From:   Hector Martin <marcan@...can.st>
To:     Kohei Tarumizu <tarumizu.kohei@...itsu.com>,
        catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, gregkh@...uxfoundation.org,
        rafael@...nel.org, mchehab+huawei@...nel.org, eugenis@...gle.com,
        tony.luck@...el.com, pcc@...gle.com, peterz@...radead.org,
        marcos@...a.pet, conor.dooley@...rochip.com,
        nicolas.ferre@...rochip.com, linus.walleij@...aro.org,
        arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 0/8] Add hardware prefetch control driver for A64FX and
 x86

On 18/05/2022 15.30, Kohei Tarumizu wrote:
> This patch series add sysfs interface to control CPU's hardware
> prefetch behavior for performance tuning from userspace for the
> processor A64FX and x86 (on supported CPU).
> 

[snip]

> In pattern A, a change of dist at L1 has a larger effect. On the other
> hand, in pattern B, the change of dist at L2 has a larger effect.
> As described above, the optimal dist combination depends on the
> characteristics of the application. Therefore, such a sysfs interface
> is useful for performance tuning.

If this is something to be tuned for specific applications, shouldn't it
be a prctl or similar and part of process context, so different
applications can use different settings (or even a single application
depending on what it's doing)? Especially if writing those sysregs/MSRs
is cheap.

In particular, configuring things separately for different cores feels
strange. You'd then have to pin applications to specific cores to get
the benefits, and wouldn't be able to optimize for multiple applications
running simultaneously that need different prefetch behavior if they
share cores.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub