perf/x86/pebs: Fix handling of PEBS buffer overflows

This patch solves a race condition between PEBS and the PMU handler.

In case multiple PEBS events are sampled at the same time,
it is possible to have GLOBAL_STATUS bit 62 set indicating
PEBS buffer overflow and also seeing at most 3 PEBS counters
having their bits set in the status register. This is a sign
that there was at least one PEBS record pending at the time
of the PMU interrupt. PEBS counters must only be processed
via the drain_pebs() calls, and not via the regular sample
processing loop coming after that the function, otherwise
phony regular samples may be generated in the sampling buffer
not marked with the EXACT tag.

Another possibility is to have one PEBS event and at least
one non-PEBS event whic hoverflows while PEBS has armed. In this
case, bit 62 of GLOBAL_STATUS will not be set, yet the overflow
status bit for the PEBS counter will be on Skylake.

To avoid this problem, we systematically ignore the PEBS-enabled
counters from the GLOBAL_STATUS mask and we always process PEBS
events via drain_pebs().

The problem manifested itself by having non-exact samples when
sampling only PEBS events, i.e., the PERF_SAMPLE_RECORD would
not have the EXACT flag set.

Note that this problem is only present on Skylake processor.
This fix is harmless on older processors.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/1482395366-8992-1-git-send-email-eranian@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/events/intel/core.c

@@ -2110,6 +2110,27 @@ again:
 		    GLOBAL_STATUS_LBRS_FROZEN);
 	if (!status)
 		goto done;
+	/*
+	 * In case multiple PEBS events are sampled at the same time,
+	 * it is possible to have GLOBAL_STATUS bit 62 set indicating
+	 * PEBS buffer overflow and also seeing at most 3 PEBS counters
+	 * having their bits set in the status register. This is a sign
+	 * that there was at least one PEBS record pending at the time
+	 * of the PMU interrupt. PEBS counters must only be processed
+	 * via the drain_pebs() calls and not via the regular sample
+	 * processing loop coming after that the function, otherwise
+	 * phony regular samples may be generated in the sampling buffer
+	 * not marked with the EXACT tag. Another possibility is to have
+	 * one PEBS event and at least one non-PEBS event whic hoverflows
+	 * while PEBS has armed. In this case, bit 62 of GLOBAL_STATUS will
+	 * not be set, yet the overflow status bit for the PEBS counter will
+	 * be on Skylake.
+	 *
+	 * To avoid this problem, we systematically ignore the PEBS-enabled
+	 * counters from the GLOBAL_STATUS mask and we always process PEBS
+	 * events via drain_pebs().
+	 */
+	status &= ~cpuc->pebs_enabled;
 
 	/*
 	 * PEBS overflow sets bit 62 in the global status register
@@ -2117,15 +2138,6 @@ again:
 	if (__test_and_clear_bit(62, (unsigned long *)&status)) {
 		handled++;
 		x86_pmu.drain_pebs(regs);
-		/*
-		 * There are cases where, even though, the PEBS ovfl bit is set
-		 * in GLOBAL_OVF_STATUS, the PEBS events may also have their
-		 * overflow bits set for their counters. We must clear them
-		 * here because they have been processed as exact samples in
-		 * the drain_pebs() routine. They must not be processed again
-		 * in the for_each_bit_set() loop for regular samples below.
-		 */
-		status &= ~cpuc->pebs_enabled;
 		status &= x86_pmu.intel_ctrl | GLOBAL_STATUS_TRACE_TOPAPMI;
 	}