Messages in this thread Patch in this message | | | From | Namhyung Kim <> | Subject | [PATCH v2] perf/core: Set event shadow time for inactive events too | Date | Tue, 30 Nov 2021 20:58:07 -0800 |
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From: Namhyung Kim <namhyung@google.com>
While f79256532682 ("perf/core: fix userpage->time_enabled of inactive events") fixed this problem for user rdpmc usage, bperf (perf stat with BPF) still has the same problem that accessing inactive perf events from BPF using bpf_perf_event_read_value().
You can reproduce this problem easily. As this is about a small window with multiplexing, we need a large number of events and short duration like below:
# perf stat -a -v --bpf-counters -e instructions,branches,branch-misses \ -e cache-references,cache-misses,bus-cycles,ref-cycles,cycles sleep 0.1
Control descriptor is not initialized instructions: 19616489 431324015 360374366 branches: 3685346 417640114 344175443 branch-misses: 75714 404089360 336145421 cache-references: 438667 390474289 327444074 cache-misses: 49279 349333164 272835067 bus-cycles: 631887 283423953 165164214 ref-cycles: 2578771111104847872 18446744069443110306 182116355 cycles: 1785221016051271680 18446744071682768912 115821694
Performance counter stats for 'system wide':
19,616,489 instructions # 0.00 insn per cycle ( 83.55%) 3,685,346 branches ( 82.41%) 75,714 branch-misses # 2.05% of all branches ( 83.19%) 438,667 cache-references ( 83.86%) 49,279 cache-misses # 11.234 % of all cache refs ( 78.10%) 631,887 bus-cycles ( 58.27%) 2,578,771,111,104,847,872 ref-cycles (0.00%) 1,785,221,016,051,271,680 cycles (0.00%)
0.010824702 seconds time elapsed
As you can see, it shows invalid values for the last two events. The -v option shows that the enabled time is way bigger than the running time. So it scaled the counter values using the ratio between the two and resulted in that. This problem can get worse if users want no-aggregation or cgroup aggregation with a small interval.
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. Later, the shadow time is set and the function starts to return a valid time. At the moment, the recent value is smaller than before so the delta in the bperf can be negative.
I think we need to set the shadow time even the events are inactive so that BPF programs (or other potential users) can see valid time values anytime.
Cc: Song Liu <songliubraving@fb.com> Signed-off-by: Namhyung Kim <namhyung@kernel.org> --- v2) rebased to tip/perf/core
kernel/events/core.c | 29 ++++++++++++++--------------- 1 file changed, 14 insertions(+), 15 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 3b3297a57228..be37f830f51c 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3707,27 +3707,26 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx, return 0; } -static inline bool event_update_userpage(struct perf_event *event) +static inline void group_update_event_time(struct perf_event *group_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); + struct perf_event *event; + struct perf_event_context *ctx = group_event->ctx; - return true; -} + perf_event_update_time(group_event); + perf_set_shadow_time(group_event, ctx); -static inline void group_update_userpage(struct perf_event *group_event) -{ - struct perf_event *event; + for_each_sibling_event(event, group_event) { + perf_event_update_time(event); + perf_set_shadow_time(event, ctx); + } - if (!event_update_userpage(group_event)) + if (likely(!atomic_read(&group_event->mmap_count))) return; + perf_event_update_userpage(group_event); + for_each_sibling_event(event, group_event) - event_update_userpage(event); + perf_event_update_userpage(event); } static int merge_sched_in(struct perf_event *event, void *data) @@ -3755,7 +3754,7 @@ static int merge_sched_in(struct perf_event *event, void *data) } else { ctx->rotate_necessary = 1; perf_mux_hrtimer_restart(cpuctx); - group_update_userpage(event); + group_update_event_time(event); } } -- 2.34.0.rc2.393.gf8c9666880-goog
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