[PATCH V2] powerpc/perf: Record counter overflow always if SAMPLE_IP is unset

[PATCH V2] powerpc/perf: Record counter overflow always if SAMPLE_IP is unset

[Athira Rajeev]

While sampling for marked events, currently we record the sample only
if the SIAR valid bit of Sampled Instruction Event Register (SIER) is
set. SIAR_VALID bit is used for fetching the instruction address from
Sampled Instruction Address Register(SIAR). But there are some usecases,
where the user is interested only in the PMU stats at each counter
overflow and the exact IP of the overflow event is not required.
Dropping SIAR invalid samples will fail to record some of the counter
overflows in such cases.

Example of such usecase is dumping the PMU stats (event counts)
after some regular amount of instructions/events from the userspace
(ex: via ptrace). Here counter overflow is indicated to userspace via
signal handler, and captured by monitoring and enabling I/O
signaling on the event file descriptor. In these cases, we expect to
get sample/overflow indication after each specified sample_period.

Perf event attribute will not have PERF_SAMPLE_IP set in the
sample_type if exact IP of the overflow event is not requested. So
while profiling if SAMPLE_IP is not set, just record the counter overflow
irrespective of SIAR_VALID check.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
---
Changes in v2:
-- Changed the approach to include PERF_SAMPLE_IP
   condition while checking siar_valid as Suggested by
   Michael Ellerman.

 arch/powerpc/perf/core-book3s.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 28206b1fe172..0ddbe33798ce 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -2149,7 +2149,17 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
 			left += period;
 			if (left <= 0)
 				left = period;
-			record = siar_valid(regs);
+
+			/*
+			 * If address is not requested in the sample
+			 * via PERF_SAMPLE_IP, just record that sample
+			 * irrespective of SIAR valid check.
+			 */
+			if (event->attr.sample_type & PERF_SAMPLE_IP)
+				record = siar_valid(regs);
+			else
+				record = 1;
+
 			event->hw.last_period = event->hw.sample_period;
 		}
 		if (left < 0x80000000LL)
@@ -2167,9 +2177,10 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
 	 * MMCR2. Check attr.exclude_kernel and address to drop the sample in
 	 * these cases.
 	 */
-	if (event->attr.exclude_kernel && record)
-		if (is_kernel_addr(mfspr(SPRN_SIAR)))
-			record = 0;
+	if (event->attr.exclude_kernel &&
+			(event->attr.sample_type & PERF_SAMPLE_IP) &&
+			is_kernel_addr(mfspr(SPRN_SIAR)))
+		record = 0;
 
 	/*
 	 * Finally record data if requested.
-- 
1.8.3.1

Re: [PATCH V2] powerpc/perf: Record counter overflow always if SAMPLE_IP is unset

[Michael Ellerman]

On Fri, 5 Feb 2021 04:14:52 -0500, Athira Rajeev wrote:
> While sampling for marked events, currently we record the sample only
> if the SIAR valid bit of Sampled Instruction Event Register (SIER) is
> set. SIAR_VALID bit is used for fetching the instruction address from
> Sampled Instruction Address Register(SIAR). But there are some usecases,
> where the user is interested only in the PMU stats at each counter
> overflow and the exact IP of the overflow event is not required.
> Dropping SIAR invalid samples will fail to record some of the counter
> overflows in such cases.
> 
> [...]

Applied to powerpc/next.

[1/1] powerpc/perf: Record counter overflow always if SAMPLE_IP is unset
      https://git.kernel.org/powerpc/c/d137845c973147a22622cc76c7b0bc16f6206323

cheers