Re: [PATCH v3] perf/core: Set event shadow time for inactive events too

From: Peter Zijlstra
Date: Thu Dec 09 2021 - 06:35:03 EST


On Thu, Dec 09, 2021 at 12:26:32PM +0100, Peter Zijlstra wrote:
> On Sun, Dec 05, 2021 at 02:48:43PM -0800, Namhyung Kim wrote:
>
> > Actually 18446744069443110306 is 0xffffffff01b345a2 so it seems to
> > have a negative enabled time. In fact, bperf keeps values returned by
> > bpf_perf_event_read_value() which calls perf_event_read_local(), and
> > accumulates delta between two calls. When event->shadow_ctx_time is
> > not set, it'd return invalid enabled time which is bigger than normal.
>
> *that*, how does it happen that shadow_time isn't set? It should be last
> set when the event switches to INACTIVE, no? At which point the logic in
> perf_event_read_local() should make @enabled move forward while @running
> stays put.
>
> Let me go rummage around a bit... either I'm missing something obvious
> or something's smelly.

How's this then?

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 7b7525e9155f..82baa8bdfc86 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -686,9 +686,7 @@ 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,
- * or when ctx_sched_in failed to schedule the event because we
- * run out of PMC.
+ * context time as it was when the event was last scheduled in.
*
* 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 28aaeacdaea1..20637b7f420c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -640,6 +640,9 @@ __perf_update_times(struct perf_event *event, u64 now, u64 *enabled, u64 *runnin
*running += delta;
}

+static void perf_set_shadow_time(struct perf_event *event,
+ struct perf_event_context *ctx);
+
static void perf_event_update_time(struct perf_event *event)
{
u64 now = perf_event_time(event);
@@ -647,6 +650,7 @@ static void perf_event_update_time(struct perf_event *event)
__perf_update_times(event, now, &event->total_time_enabled,
&event->total_time_running);
event->tstamp = now;
+ perf_set_shadow_time(event, event->ctx);
}

static void perf_event_update_sibling_time(struct perf_event *leader)
@@ -2552,8 +2556,6 @@ event_sched_in(struct perf_event *event,

perf_pmu_disable(event->pmu);

- perf_set_shadow_time(event, ctx);
-
perf_log_itrace_start(event);

if (event->pmu->add(event, PERF_EF_START)) {
@@ -3707,29 +3709,6 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx,
return 0;
}

-static inline bool event_update_userpage(struct perf_event *event)
-{
- if (likely(!atomic_read(&event->mmap_count)))
- return false;
-
- perf_event_update_time(event);
- perf_set_shadow_time(event, event->ctx);
- perf_event_update_userpage(event);
-
- return true;
-}
-
-static inline void group_update_userpage(struct perf_event *group_event)
-{
- struct perf_event *event;
-
- if (!event_update_userpage(group_event))
- return;
-
- for_each_sibling_event(event, group_event)
- event_update_userpage(event);
-}
-
static int merge_sched_in(struct perf_event *event, void *data)
{
struct perf_event_context *ctx = event->ctx;
@@ -3748,15 +3727,14 @@ static int merge_sched_in(struct perf_event *event, void *data)
}

if (event->state == PERF_EVENT_STATE_INACTIVE) {
- *can_add_hw = 0;
if (event->attr.pinned) {
perf_cgroup_event_disable(event, ctx);
perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
- } else {
- ctx->rotate_necessary = 1;
- perf_mux_hrtimer_restart(cpuctx);
- group_update_userpage(event);
}
+
+ *can_add_hw = 0;
+ ctx->rotate_necessary = 1;
+ perf_mux_hrtimer_restart(cpuctx);
}

return 0;
@@ -6349,7 +6327,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
ring_buffer_attach(event, rb);

perf_event_update_time(event);
- perf_set_shadow_time(event, event->ctx);
perf_event_init_userpage(event);
perf_event_update_userpage(event);
} else {