[PATCH V2] powerpc/perf: Fix task context setting for trace imc

[PATCH V2] powerpc/perf: Fix task context setting for trace imc

[Athira Rajeev]

Trace IMC (In-Memory collection counters) in powerpc is
useful for application level profiling. For trace_imc,
presently task context (task_ctx_nr) is set to
perf_hw_context. But perf_hw_context is to be used for
cpu PMU. So for trace_imc, even though it is per thread
PMU, it is preferred to use sw_context inorder to be able
to do application level monitoring. Hence change the
task_ctx_nr to use perf_sw_context.

Fixes: 012ae244845f ("powerpc/perf: Trace imc PMU functions")
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
---
Changelog:
v1 -> v2:
 Added comment in code on why perf_sw_context is used.
Notes:
 trace_imc_event_init currently uses context as
 perf_hw_context. But ideally there can only be a single
 PMU for perf_hw_context events which is core PMU.
 Reference:
 commit 26657848502b ("perf/core: Verify we have a single perf_hw_context PMU")
 Reason for using "perf_sw_context" instead of invalid_context
 is that, task level monitoring is restricted with
 invalid_context.

 arch/powerpc/perf/imc-pmu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
index e106909ff9c3..8fe57601e61d 100644
--- a/arch/powerpc/perf/imc-pmu.c
+++ b/arch/powerpc/perf/imc-pmu.c
@@ -1457,7 +1457,13 @@ static int trace_imc_event_init(struct perf_event *event)
 
 	event->hw.idx = -1;
 
-	event->pmu->task_ctx_nr = perf_hw_context;
+	/*
+	 * There can only be a single PMU for
+	 * perf_hw_context events which is assigned
+	 * to core PMU. Hence use "perf_sw_context" for
+	 * trace_imc.
+	 */
+	event->pmu->task_ctx_nr = perf_sw_context;
 	event->destroy = reset_global_refc;
 	return 0;
 }
-- 
2.33.0

Re: [PATCH V2] powerpc/perf: Fix task context setting for trace imc

[Michael Ellerman]

On Wed, 2 Feb 2022 09:48:37 +0530, Athira Rajeev wrote:
> Trace IMC (In-Memory collection counters) in powerpc is
> useful for application level profiling. For trace_imc,
> presently task context (task_ctx_nr) is set to
> perf_hw_context. But perf_hw_context is to be used for
> cpu PMU. So for trace_imc, even though it is per thread
> PMU, it is preferred to use sw_context inorder to be able
> to do application level monitoring. Hence change the
> task_ctx_nr to use perf_sw_context.
> 
> [...]

Applied to powerpc/next.

[1/1] powerpc/perf: Fix task context setting for trace imc
      https://git.kernel.org/powerpc/c/0198322379c25215b2778482bf1221743a76e2b5

cheers