linux-kernel - [PATCH] perf/core: fix userpage->time

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Date:   Tue, 21 Sep 2021 18:17:15 -0700
From:   Song Liu <songliubraving@...com>
To:     <linux-kernel@...r.kernel.org>
CC:     <acme@...nel.org>, <peterz@...radead.org>, <mingo@...hat.com>,
        <kernel-team@...com>, <eranian@...gle.com>,
        Song Liu <songliubraving@...com>,
        Lucian Grijincu <lucian@...com>
Subject: [PATCH] perf/core: fix userpage->time_enabled of inactive events

Users of rdpmc rely on the mmapped user page to calculate accurate
time_enabled. Currently, userpage->time_enabled is only updated when the
event is added to the pmu. As a result, inactive event (due to counter
multiplexing) does not have accurate userpage->time_enabled. This can
be reproduced with something like:

   /* open 20 task perf_event "cycles", to create multiplexing */

   fd = perf_event_open();  /* open task perf_event "cycles" */
   userpage = mmap(fd);     /* use mmap and rdmpc */

   while (true) {
     time_enabled_mmap = xxx; /* use logic in perf_event_mmap_page */
     time_enabled_read = read(fd).time_enabled;
     if (time_enabled_mmap > time_enabled_read)
         BUG();
   }

Fix this by updating userpage for inactive events in ctx_sched_in.

Reported-and-tested-by: Lucian Grijincu <lucian@...com>
Signed-off-by: Song Liu <songliubraving@...com>
---
 include/linux/perf_event.h |  4 +++-
 kernel/events/core.c       | 26 ++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2d510ad750edc..4aa52f7a48c16 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -683,7 +683,9 @@ struct perf_event {
 	/*
 	 * timestamp shadows the actual context timing but it can
 	 * be safely used in NMI interrupt context. It reflects the
-	 * context time as it was when the event was last scheduled in.
+	 * context time as it was when the event was last scheduled in,
+	 * or when ctx_sched_in failed to schedule the event because we
+	 * run out of PMC.
 	 *
 	 * ctx_time already accounts for ctx->timestamp. Therefore to
 	 * compute ctx_time for a sample, simply add perf_clock().
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1cb1f9b8392e2..549ce22df7bc7 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3766,6 +3766,15 @@ ctx_flexible_sched_in(struct perf_event_context *ctx,
 			   merge_sched_in, &can_add_hw);
 }
 
+static inline void
+perf_event_update_inactive_userpage(struct perf_event *event,
+				    struct perf_event_context *ctx)
+{
+	perf_event_update_time(event);
+	perf_set_shadow_time(event, ctx);
+	perf_event_update_userpage(event);
+}
+
 static void
 ctx_sched_in(struct perf_event_context *ctx,
 	     struct perf_cpu_context *cpuctx,
@@ -3807,6 +3816,23 @@ ctx_sched_in(struct perf_event_context *ctx,
 	/* Then walk through the lower prio flexible groups */
 	if (is_active & EVENT_FLEXIBLE)
 		ctx_flexible_sched_in(ctx, cpuctx);
+
+	/*
+	 * Update userpage for inactive events. This is needed for accurate
+	 * time_enabled.
+	 */
+	if (unlikely(ctx->rotate_necessary)) {
+		struct perf_event *event;
+
+		perf_event_groups_for_each(event, &ctx->pinned_groups) {
+			if (event->state == PERF_EVENT_STATE_INACTIVE)
+				perf_event_update_inactive_userpage(event, ctx);
+		}
+		perf_event_groups_for_each(event, &ctx->flexible_groups) {
+			if (event->state == PERF_EVENT_STATE_INACTIVE)
+				perf_event_update_inactive_userpage(event, ctx);
+		}
+	}
 }
 
 static void cpu_ctx_sched_in(struct perf_cpu_context *cpuctx,
-- 
2.30.2