linux-kernel - Re: [PATCH] perf: add 'perf bench syscall'

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Date:	Tue, 21 Jun 2016 09:31:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	clark@...hat.com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] perf: add 'perf bench syscall'

On Jun 21, 2016 7:55 AM, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
>
> On Mon, Jun 20, 2016 at 12:16:22PM -0700, Andy Lutomirski wrote:
> > On Mon, Jun 20, 2016 at 11:00 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > >
> > > From: Josh Poimboeuf <jpoimboe@...hat.com>
> > > Subject: [PATCH] perf: add 'perf bench syscall'
> > >
> > > Add a basic 'perf bench syscall' benchmark which does a getppid() system
> > > call in a tight loop.
> > >
> >
> > My one suggestion would be to use a different syscall than getppid(),
> > as getppid() can have unusual performance characteristics on kernels
> > with pid namespaces enabled.  I'm a fan of prctl(-1, 0, 0, 0, 0).
>
> Hm, can you elaborate on the unusual performance characteristics of
> getppid()?  The code seems pretty minimal.

task_this_vnr -> pid_vnr -> pid_nr_ns

There's probably something better than prctl, though.

--Andy