lkml.org 
[lkml]   [2021]   [Dec]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v3] perf/core: Set event shadow time for inactive events too
Date


> On Dec 5, 2021, at 2:48 PM, Namhyung Kim <namhyung@kernel.org> wrote:
>
> While commit 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>

Acked-by: Song Liu <song@kernel.org>

\
 
 \ /
  Last update: 2021-12-07 00:12    [W:0.112 / U:0.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site