linux-kernel - Re: perf: question about event scheduler

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Message-ID: <CABPqkBTeQeR3n_KDWffWmz0AxL8+ynfvv0tCSjPP8aCh81dgPg@mail.gmail.com>
Date:	Mon, 29 Jul 2013 11:23:46 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: perf: question about event scheduler

On Tue, Jul 23, 2013 at 11:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Jul 23, 2013 at 10:13:33AM +0200, Stephane Eranian wrote:
> > Hi,
> >
> > I am looking at ctx_pinned_sched_in() and
> > ctx_flexible_sched_in() and I am trying to
> > understand the difference of treatment in
> > case of errors for the two classes of events
> > (pinned vs. flexible).
> >
> > For pinned events, when a group fails to
> > schedule in, the code goes on to the next
> > group and therefore walks the entire list
> > for each scheduler invocation.
> >
> > For flexible events, when a group fails,
> > the loop aborts and no subsequent group
> > is tried.
> >
> > I am trying to understand the motivation for
> > this difference here.
> >
> > If I recall, the abort is here to limit malicious
> > DoS where a malicious user would provide
> > an arbitrary long list of events, hogging the kernel.
> > But in the case of pinned events, this is ignored
> > because to create such events one needs to be
> > root in the first place.
> >
> > Am I getting this right?
>
> Whee, long time ago. I think the biggest reason is that pinned events
> should always be scheduled. Not being able to schedule a pinned event is
> an error. But yes, that and the fact that they're root only.
>
>
Ok, that's what I thought then.
Thanks,
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