linux-kernel - Re: [PATCH] perf/x86: Fix overlap counter scheduling bug

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Date:   Thu, 10 Nov 2016 09:00:13 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Robert Richter <rric@...nel.org>,
        "Liang, Kan" <kan.liang@...el.com>,
        Andi Kleen <andi@...stfloor.org>, Jiri Olsa <jolsa@...nel.org>,
        Vince Weaver <vince@...ter.net>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf/x86: Fix overlap counter scheduling bug


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Nov 09, 2016 at 03:25:15PM +0100, Robert Richter wrote:
> > On 08.11.16 19:27:39, Peter Zijlstra wrote:
> > > The comment with EVENT_CONSTRAINT_OVERLAP states: "This is the case if
> > > the counter mask of such an event is not a subset of any other counter
> > > mask of a constraint with an equal or higher weight".
> > > 
> > > Esp. that latter part is of interest here I think, our overlapping mask
> > > is 0x0e, that has 3 bits set and is the highest weight mask in on the
> > > PMU, therefore it will be placed last. Can we still create a scenario
> > > where we would need to rewind that?
> > > 
> > > The scenario for AMD Fam15h is we're having masks like:
> > > 
> > > 	0x3F -- 111111
> > > 	0x38 -- 111000
> > > 	0x07 -- 000111
> > > 
> > > 	0x09 -- 001001
> > > 
> > > And we mark 0x09 as overlapping, because it is not a direct subset of
> > > 0x38 or 0x07 and has less weight than either of those. This means we'll
> > > first try and place the 0x09 event, then try and place 0x38/0x07 events.
> > > Now imagine we have:
> > > 
> > > 	3 * 0x07 + 0x09
> > > 
> > > and the initial pick for the 0x09 event is counter 0, then we'll fail to
> > > place all 0x07 events. So we'll pop back, try counter 4 for the 0x09
> > > event, and then re-try all 0x07 events, which will now work.
> > > 
> > > 
> > > 
> > > But given, that in the uncore case, the overlapping event is the
> > > heaviest mask, I don't think this can happen. Or did I overlook
> > > something.... takes a bit to page all this back in.
> > 
> > Right, IMO 0xE mask may not be marked as overlapping. It is placed
> > last and if there is no space left we are lost. There is no other
> > combination or state we could try then. So the fix is to remove the
> > overlapping bit for that counter, the state is then never saved.
> > 
> > This assumes there are no other counters than 0x3 and 0xc that overlap
> > with 0xe. It becomes a bit tricky if there is another counter with the
> > same or higher weight that overlaps with 0xe, e.g. 0x7.
> 
> As per a prior mail, the masks on the PMU in question are:
> 
>  0x01 - 0001
>  0x03 - 0011
>  0x0e - 1110
>  0x0c - 1100
> 
> But since all the masks that have overlap (0xe -> {0xc,0x3}) and (0x3 ->
> 0x1) are of heavier weight, it should all work out I think.
> 
> So yes, something like the below (removing the OVERLAP bit) looks like
> its sufficient.

Would it be possible to also add debug code (or some other mechanism) to disallow 
such buggy EVENT_CONSTRAINT_OVERLAP() definitions?

Thanks,

	Ingo