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

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Message-ID: <20130723094832.GV27075@twins.programming.kicks-ass.net>
Date:	Tue, 23 Jul 2013 11:48:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
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 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.


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