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

Peter Zijlstra
2016-11-09T15:51:55Z

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.

---
arch/x86/events/intel/uncore_snbep.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/uncore_snbep.c b/arch/x86/events/intel/uncore_snbep.c
index 272427700d48..e6832be714bc 100644
--- a/arch/x86/events/intel/uncore_snbep.c
+++ b/arch/x86/events/intel/uncore_snbep.c
@@ -669,7 +669,7 @@ static struct event_constraint snbep_uncore_cbox_constraints[] = {
UNCORE_EVENT_CONSTRAINT(0x1c, 0xc),
UNCORE_EVENT_CONSTRAINT(0x1d, 0xc),
UNCORE_EVENT_CONSTRAINT(0x1e, 0xc),
- EVENT_CONSTRAINT_OVERLAP(0x1f, 0xe, 0xff),
+ UNCORE_EVENT_CONSTRAINT(0x1f, 0xe),
UNCORE_EVENT_CONSTRAINT(0x21, 0x3),
UNCORE_EVENT_CONSTRAINT(0x23, 0x3),
UNCORE_EVENT_CONSTRAINT(0x31, 0x3),