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Message-Id: <EEA8D03E-14A7-4A8B-8C99-B68CACEC25D2@linux.vnet.ibm.com>
Date: Wed, 29 Sep 2021 22:34:41 +0530
From: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
To: Song Liu <songliubraving@...com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, kernel-team@...com,
Stephane Eranian <eranian@...gle.com>,
Lucian Grijincu <lucian@...com>,
Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
kajoljain <kjain@...ux.ibm.com>
Subject: Re: [PATCH v2] perf/core: fix userpage->time_enabled of inactive
events
> On 24-Sep-2021, at 6:58 AM, Song Liu <songliubraving@...com> wrote:
>
> 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 merge_sched_in.
>
> Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 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 | 49 ++++++++++++++++++++++++++++++++++----
> 2 files changed, 48 insertions(+), 5 deletions(-)
>
> 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..d73f986eef7b3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3707,6 +3707,46 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx,
> return 0;
> }
>
> +static inline bool event_update_userpage(struct perf_event *event)
> +{
> + /*
> + * Checking mmap_count to avoid unnecessary work. This does leave a
> + * corner case: if the event is enabled before mmap(), the first
> + * time the event gets scheduled is via:
> + *
> + * __perf_event_enable (or __perf_install_in_context)
> + * -> ctx_resched
> + * -> perf_event_sched_in
> + * -> ctx_sched_in
> + *
> + * with mmap_count of 0, so we will skip here. As a result,
> + * userpage->offset is not accurate after mmap and before the
> + * first rotation.
> + *
> + * To avoid the discrepancy of this window, the user space should
> + * mmap the event before enabling it.
> + */
> + 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;
> @@ -3725,14 +3765,15 @@ 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);
> }
Another optimisation that is possible in merge_sched_in is we can avoid calling "perf_mux_hrtimer_restart" multiple times if already rotate_necessary is set for that context. Even though "perf_mux_hrtimer_restart" will just return if hrtimer is already active, we could avoid the overhead of calling this function multiple times if there are many groups.
Something like below:
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0c000cb01eeb..26eae79bd723 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3731,8 +3731,10 @@ static int merge_sched_in(struct perf_event *event, void *data)
}
*can_add_hw = 0;
- ctx->rotate_necessary = 1;
- perf_mux_hrtimer_restart(cpuctx);
+ if (!ctx->rotate_necessary) {
+ ctx->rotate_necessary = 1;
+ perf_mux_hrtimer_restart(cpuctx);
+ }
}
return 0;
Thanks,
Athira Rajeev
>
> return 0;
> --
> 2.30.2
>
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