linux-kernel - Re: perf code using smp_processor_id() in preemptible [00000000] code

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Date:	Fri, 15 Nov 2013 13:33:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: perf code using smp_processor_id() in preemptible [00000000] code

On Fri, Nov 15, 2013 at 04:10:51PM +0400, Cyrill Gorcunov wrote:
> On Fri, Nov 15, 2013 at 12:51:50PM +0100, Peter Zijlstra wrote:
> > ok, this will make the error go away, but what about the semantics of
> > the case? Does it really matter for the grouping on which cpu we compute
> > it? That is can we end up with a different group for one cpu as for
> > another?
> > 
> > Or do we simply need a coherent single cpu to do the computation with?
> > In which case raw_smp_processor_id() would also suffice.
> > 
> > If we can indeed get a different result depending on which cpu we do the
> > computation, then things are broken, because it might be a task group
> > we're building which has to be able to migrate around with the task.
> 
> The events are sensitive to which cpu they're scheduled to execute on
> (if HT is turned on, we need to setup thread bit in register).
> As far as I understand once events are assigned to cpu_hw_events
> they are executing on this cpu, when tasks are migrated to another
> cpu, they're re-scheduled. Or I miss something obvious here?

No this is correct, but that is simply about event encoding, right?

The situation we should be avoiding is:

 {x, y, z}

being a valid event group on ht0 but an invalid group for ht1.

So the whole fake_cpuc / validate_{event,group} code that triggered this
isn't actually scheduling them, its testing to see if all the provided
events could possibly be scheduled together -- and we would want to
avoid giving a sibling dependent answer here.
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