[RFC][PATCH 4/7] perf: Fix scaling vs enable_on_exec

[Peter Zijlstra <peterz@infradead.org>]

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Oleg reported that enable_on_exec results in weird scale factors.

The recent commit 3e349507d12d ("perf: Fix perf_enable_on_exec() event
scheduling") caused this by moving task_ctx_sched_out() from before
__perf_event_mask_enable() to after it.

The overlooked concequence of that change is that task_ctx_sched_out()
would update the ctx time fields, and now __perf_event_mask_enable()
uses stale time.

Fix this by adding an explicit time update.

While looking at this, I also found that we need an ctx->is_active
check in perf_install_in_context().

XXX: does this actually fix the reported issue? I'm not sure what the
reproduction case is. Also an earlier version made Jiri's machine
explode -- something I've not managed to reproduce either.

Fixes: 3e349507d12d ("perf: Fix perf_enable_on_exec() event scheduling")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2170,12 +2170,12 @@ perf_install_in_context(struct perf_even
 		raw_spin_unlock_irq(&ctx->lock);
 		return;
 	}
-	update_context_time(ctx);
-	/*
-	 * Update cgrp time only if current cgrp matches event->cgrp.
-	 * Must be done before calling add_event_to_ctx().
-	 */
-	update_cgrp_time_from_event(event);
+
+	if (ctx->is_active) {
+		update_context_time(ctx);
+		update_cgrp_time_from_event(event);
+	}
+
 	add_event_to_ctx(event, ctx);
 	raw_spin_unlock_irq(&ctx->lock);
 
@@ -3122,6 +3122,12 @@ static void perf_event_enable_on_exec(in
 
 	cpuctx = __get_cpu_context(ctx);
 	perf_ctx_lock(cpuctx, ctx);
+
+	if (ctx->is_active) {
+		update_context_time(ctx);
+		update_cgrp_time_from_cpuctx(cpuctx);
+	}
+
 	list_for_each_entry(event, &ctx->event_list, event_entry)
 		enabled |= event_enable_on_exec(event, ctx);