linux-kernel - RE: [RFC PATCH v2 5/5] docs: ABI: Add sysfs documentation interface of hardware prefetch driver

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Date:   Tue, 9 Nov 2021 09:41:23 +0000
From:   "tarumizu.kohei@...itsu.com" <tarumizu.kohei@...itsu.com>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH v2 5/5] docs: ABI: Add sysfs documentation interface
 of hardware prefetch driver

> Ahh, so you really do intend the l2 directory to be for *all* the L2
> prefetchers?

Yes, we intend to create the l2 directory for *all* the L2 prefetchers
(i.e. "L2 Hardware Prefetcher Disable" and "L2 Adjacent Cache Line
Prefetcher Disable). 

> I guess that's OK, but will folks ever want to do "L2
> Hardware Prefetcher Disable", but not "L2 Adjacent Cache Line Prefetcher
> Disable"?

There are people who actually tested the performance improvement[1].

[1]https://github.com/xmrig/xmrig/issues/1433#issuecomment-572126184

In this report, write 5 to MSR 0x1a4 (i.e. "L2 Hardware Prefetcher
Disable", but not "L2 Adjacent Cache Line Prefetcher Disable")
on i7-5930K for best performance. If such tuning is possible, it may
be useful for some people.

We describe how to deal these parameters in our sysfs interface at
"[RFC & Future plan]" section in the cover letter(0/5), but we can't
come up with any good ideas.

We thought that the sysfs interface should be generic and common,
and avoid showing architecture-dependent specifications.

We have considered the Proposal B that multiple hardware prefetch
types in one enable attribute file at above section. However, in
order to use it, we have to know the register specification, so we
think it is not appropriate.

Do you have any idea how to represent architecture-dependent
specifications in sysfs interface?