linux-kernel - Re: [PATCH V6 11/14] perf/x86/intel: Disable sample-read the slots and metrics events

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Message-ID: <20200721131011.GY10769@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Jul 2020 15:10:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH V6 11/14] perf/x86/intel: Disable sample-read the slots
 and metrics events

On Fri, Jul 17, 2020 at 07:05:51AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> Users fail to sample-read the slots and metrics events, e.g.,
> perf record -e '{slots, topdown-retiring}:S'.
> 
> When reading the metrics event, the fixed counter 3 (slots) has to be
> reset, which impacts the sampling of the slots event.
> 
> Add a specific validate_group() support to reject the case and error out
> for Ice Lake.
> 
> An alternative fix may unconditionally disable slots sampling, but it's
> not a decent fix. Users may want to only sample the slot events
> without the topdown metrics events.
> 
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>

I'm confused by this; it doesn't make sense.

Should not patch 7 have something like the below instead?

Also, I think there is a bug when we create a group like this and then
kill the leader, in that case the core code will 'promote' the sibling
metric events to their own individual events, see perf_group_detach().

We need additional code to move those events into unrecoverable ERROR
state. A new group_caps flag could indicate this promotion isn't
allowed.

--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3441,8 +3441,22 @@ static int intel_pmu_hw_config(struct pe
 	 * A flag PERF_X86_EVENT_TOPDOWN is applied for the case.
 	 */
 	if (x86_pmu.intel_cap.perf_metrics && is_topdown_event(event)) {
-		if (is_metric_event(event) && is_sampling_event(event))
-			return -EINVAL;
+
+		if (is_metric_event(event)) {
+			struct perf_event *leader = event->group_leader;
+
+			if (is_sampling_event(event))
+				return -EINVAL;
+
+			if (leader == event)
+				return -EINVAL;
+
+			if (!is_slots_event(leader))
+				return -EINVAL;
+
+			if (is_sampling_event(leader))
+				return -EINVAL;
+		}
 
 		if (!is_sampling_event(event)) {
 			if (event->attr.config1 != 0)