linux-hardening - Re: [PATCH] selftests/seccomp: Pin benchmark to single CPU

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Message-ID: <202402060302.C9BD99F@keescook>
Date: Tue, 6 Feb 2024 03:04:32 -0800
From: Kees Cook <keescook@...omium.org>
To: Mark Brown <broonie@...nel.org>
Cc: kernel test robot <oliver.sang@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] selftests/seccomp: Pin benchmark to single CPU

On Tue, Feb 06, 2024 at 10:16:19AM +0000, Mark Brown wrote:
> On Tue, Feb 06, 2024 at 01:56:47AM -0800, Kees Cook wrote:
> 
> > +	/* Set from highest CPU down. */
> > +	for (cpu = ncores - 1; cpu >= 0; cpu--) {
> > +		CPU_ZERO_S(setsz, setp);
> > +		CPU_SET_S(cpu, setsz, setp);
> 
> Is there some particular reason to go from the highest CPU number down?
> Not that it super matters but the default would be to iterate from 0 and
> there's a comment but it just says the what not the why.

I was arbitrarily picking a direction and all the examples I could find
started at 0, so this would be more (?) out of the way. :P

Without a cpu cgroup, I can't _exclude_ the pinned CPU from other
processes, so I was pretending the last CPU will be less likely to be
used.

-- 
Kees Cook