Re: [PATCH 0/4] perf: Fix the ctx->pmu for a hybrid system

[Peter Zijlstra]

On Thu, Jun 17, 2021 at 12:23:06PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 16, 2021 at 11:55:30AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> 
> > To fix the issue, the generic perf codes have to understand the
> > supported CPU mask of a specific hybrid PMU. So it can update the
> > ctx->pmu accordingly, when a task is scheduled on a CPU which has
> > a different type of PMU from the previous CPU. The supported_cpus
> > has to be moved to the struct pmu.
> 
> Urghh.. I so hate this :-/
> 
> I *did* point you to:
> 
>   https://lore.kernel.org/lkml/20181010104559.GO5728@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> 
> when you started this whole hybrid crud, and I think that's still the
> correct thing to do.
> 
> Still, let me consider if there's a workable short-term cludge I hate
> less.

How's this? We already have x86_pmu_update_cpu_context() setting the
'correct' pmu in the cpuctx, so we can simply fold that back into the
task context.

For normal use this is a no-op.

Now I need to go audit all ctx->pmu usage :-(

---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index db4604c4c502..6a496c29ef00 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3822,9 +3822,16 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 					struct task_struct *task)
 {
 	struct perf_cpu_context *cpuctx;
-	struct pmu *pmu = ctx->pmu;
+	struct pmu *pmu;
 
 	cpuctx = __get_cpu_context(ctx);
+
+	/*
+	 * HACK; for HETEROGENOUS the task context might have switched to a
+	 * different PMU, don't bother gating this.
+	 */
+	pmu = ctx->pmu = cpuctx->ctx.pmu;
+
 	if (cpuctx->task_ctx == ctx) {
 		if (cpuctx->sched_cb_usage)
 			__perf_pmu_sched_task(cpuctx, true);