Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems

[Peter Zijlstra <peterz@infradead.org>]

OK,.. so the AMD IBS PMUs actually have perf_invalid_context.

Lemme have a proper look...


Weirdness.. perf_pmu_register() will allocate a pmu->pmu_cpu_context for
each PMU. find_pmu_context() even special cases the perf_invalid_context
to return NULL to force the allocation instead of sharing it.

So both IBS PMUs should have their own cpuctx.



In any case, I was talking about something like the below.. I hate
growing the per-task ctx array with two entries, esp. since we'll mostly
add two NULL pointer checks on every perf operation for everybody not
using IBS.

---
--- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
+++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
@@ -436,7 +436,7 @@ static void perf_ibs_read(struct perf_ev
 
 static struct perf_ibs perf_ibs_fetch = {
 	.pmu = {
-		.task_ctx_nr	= perf_invalid_context,
+		.task_ctx_nr	= perf_hw2_context,
 
 		.event_init	= perf_ibs_init,
 		.add		= perf_ibs_add,
@@ -459,7 +459,7 @@ static struct perf_ibs perf_ibs_fetch =
 
 static struct perf_ibs perf_ibs_op = {
 	.pmu = {
-		.task_ctx_nr	= perf_invalid_context,
+		.task_ctx_nr	= perf_hw3_context,
 
 		.event_init	= perf_ibs_init,
 		.add		= perf_ibs_add,
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1237,6 +1237,8 @@ enum perf_event_task_context {
 	perf_invalid_context = -1,
 	perf_hw_context = 0,
 	perf_sw_context,
+	perf_hw2_context, /* AMD IBS (fetch) */
+	perf_hw3_context, /* AMD IBS (ops)   */
 	perf_nr_task_contexts,
 };