Re: [PATCH] perf/core: Fix time tracking bug with multiplexing

[Peter Zijlstra]

On Tue, Mar 29, 2016 at 03:33:23AM +0200, Stephane Eranian wrote:
> This patch fixes a bug introduced by:
> 
> commit 3cbaa59069677920186dcf502632ca1df4329f80
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date:   Wed Feb 24 18:45:47 2016 +0100
> 
>     perf: Fix ctx time tracking by introducing EVENT_TIME

Normal quoting style is:

  3cbaa5906967 ("perf: Fix ctx time tracking by introducing EVENT_TIME")

I have the following git alias to help with that:

  one = show -s --pretty='format:%h (\"%s\")'

> The problem is that in case that the kernel enters ctx_sched_out() with the
> following state:
>    ctx->is_active=0x7 event_type=0x1
>    Call Trace:
>     [<ffffffff813ddd41>] dump_stack+0x63/0x82
>     [<ffffffff81182bdc>] ctx_sched_out+0x2bc/0x2d0
>     [<ffffffff81183896>] perf_mux_hrtimer_handler+0xf6/0x2c0
>     [<ffffffff811837a0>] ? __perf_install_in_context+0x130/0x130
>     [<ffffffff810f5818>] __hrtimer_run_queues+0xf8/0x2f0
>     [<ffffffff810f6097>] hrtimer_interrupt+0xb7/0x1d0
>     [<ffffffff810509a8>] local_apic_timer_interrupt+0x38/0x60
>     [<ffffffff8175ca9d>] smp_apic_timer_interrupt+0x3d/0x50
>     [<ffffffff8175ac7c>] apic_timer_interrupt+0x8c/0xa0
> 
> In that case, the test:
> 	if (is_active & EVENT_TIME)
> 
> will be false and the time will not be updated. Time must always be updated on
> sched out. This patch fixes the problem.

Humm, no. It breaks things like ctx_sched_out(.event_type = EVENT_ALL),
which will set ctx->is_active = 0, and then not update time.

> +++ b/kernel/events/core.c
> @@ -2447,7 +2447,7 @@ static void ctx_sched_out(struct perf_event_context *ctx,
>  
>  	is_active ^= ctx->is_active; /* changed bits */
>  
> -	if (is_active & EVENT_TIME) {
> +	if (ctx->is_active & EVENT_TIME) {
>  		/* update (and stop) ctx time */
>  		update_context_time(ctx);
>  		update_cgrp_time_from_cpuctx(cpuctx);

I think you want something like this.

---
 kernel/events/core.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 614614821f00..10ee22b8d2a8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2402,14 +2402,24 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 			cpuctx->task_ctx = NULL;
 	}
 
-	is_active ^= ctx->is_active; /* changed bits */
-
+	/*
+	 * Always update time if it was set; not only when it changes.
+	 * Otherwise we can 'forget' to update time for any but the last
+	 * context we sched out. For example:
+	 *
+	 *   ctx_sched_out(.event_type = EVENT_FLEXIBLE)
+	 *   ctx_sched_out(.event_type = EVENT_PINNED)
+	 *
+	 * would only update time for the pinned events.
+	 */
 	if (is_active & EVENT_TIME) {
 		/* update (and stop) ctx time */
 		update_context_time(ctx);
 		update_cgrp_time_from_cpuctx(cpuctx);
 	}
 
+	is_active ^= ctx->is_active; /* changed bits */
+
 	if (!ctx->nr_active || !(is_active & EVENT_ALL))
 		return;