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

Peter Zijlstra
2016-11-08T12:20:41Z

On Tue, Nov 01, 2016 at 04:44:28PM +0100, Jiri Olsa wrote:
> My fuzzer testing hits following warning in the counter scheduling code:
>
> WARNING: CPU: 0 PID: 0 at arch/x86/events/core.c:718 perf_assign_events+0x2ae/0x2c0
> Call Trace:
> <IRQ>
> dump_stack+0x68/0x93
> __warn+0xcb/0xf0
> warn_slowpath_null+0x1d/0x20
> perf_assign_events+0x2ae/0x2c0
> uncore_assign_events+0x1a7/0x250 [intel_uncore]
> uncore_pmu_event_add+0x7a/0x3c0 [intel_uncore]
> event_sched_in.isra.104+0xf6/0x2e0
> group_sched_in+0x6e/0x190
> ...
>
> The reason is that the counter scheduling code assumes
> overlap constraints with mask weight < SCHED_STATES_MAX.
>
> This assumption is broken with uncore cbox constraint
> added for snbep in:

> 3b19e4c98c03 perf/x86: Fix event constraint for SandyBridge-EP C-Box

3b19e4c98c03 ("perf/x86: Fix event constraint for SandyBridge-EP C-Box")

Is the right form.

> It's also easily triggered by running following perf command
> on snbep box:
> # perf stat -e 'uncore_cbox_0/event=0x1f/,uncore_cbox_0/event=0x1f/,uncore_cbox_0/event=0x1f/' -a
>
> Fixing this by increasing the SCHED_STATES_MAX to 3 and adding build
> check for EVENT_CONSTRAINT_OVERLAP macro.

> -#define SCHED_STATES_MAX 2
> +#define SCHED_STATES_MAX 3

Us having to increase this is ff'ing sad :/ That's seriously challenged
hardware :/

> +
> +/* Check we dont overlap beyond the states max. */
> +#define OVERLAP_CHECK(n) (!!sizeof(char[1 - 2*!!(HWEIGHT(n) > SCHED_STATES_MAX)]))
> +#define OVERLAP_HWEIGHT(n) (OVERLAP_CHECK(n)*HWEIGHT(n))

I'm not sure I get how this is correct at all. You cannot tell by a
single mask what the overlap is. You need all the masks.

The point is that that PMU has constraints like:

0x01 - 0001
0x03 - 0011
0x0e - 1110
0x0c - 1100

Which gets us a total of 4 overlapping counter masks, and that would
indeed lead to max 3 retries I think.

Now, I would much rather solve this by changing the constraint like the
below, that yields:

0x01 - 0001
0x03 - 0011

0x0c - 1100

Which is two distinct groups, only one of which has overlap. And the one
with overlap only has 2 overlapping masks, giving a max reties of 1.

diff --git a/arch/x86/events/intel/uncore_snbep.c b/arch/x86/events/intel/uncore_snbep.c
index 272427700d48..71bc348736bd 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, 0xc); /* should be 0x0e but that gives scheduling pain */
UNCORE_EVENT_CONSTRAINT(0x21, 0x3),
UNCORE_EVENT_CONSTRAINT(0x23, 0x3),
UNCORE_EVENT_CONSTRAINT(0x31, 0x3),