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Message-ID: <a530f6de-3a89-47fc-bc4e-ea5cc57ee006@linux.intel.com>
Date: Tue, 27 Aug 2024 12:32:44 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Luo Gengkun <luogengkun@...weicloud.com>, peterz@...radead.org
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
irogers@...gle.com, adrian.hunter@...el.com,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] perf/core: Fix small negative period being ignored
On 2024-08-21 9:42 a.m., Luo Gengkun wrote:
> In perf_adjust_period, we will first calculate period, and then use
> this period to calculate delta. However, when delta is less than 0,
> there will be a deviation compared to when delta is greater than or
> equal to 0. For example, when delta is in the range of [-14,-1], the
> range of delta = delta + 7 is between [-7,6], so the final value of
> delta/8 is 0. Therefore, the impact of -1 and -2 will be ignored.
> This is unacceptable when the target period is very short, because
> we will lose a lot of samples.
>
> Here are some tests and analyzes:
> before:
> # perf record -e cs -F 1000 ./a.out
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.022 MB perf.data (518 samples) ]
>
> # perf script
> ...
> a.out 396 257.956048: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.957891: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.959730: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.961545: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.963355: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.965163: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.966973: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.968785: 23 cs: ffffffff81f4eeec schedul>
> a.out 396 257.970593: 23 cs: ffffffff81f4eeec schedul>
> ...
>
> after:
> # perf record -e cs -F 1000 ./a.out
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.058 MB perf.data (1466 samples) ]
>
> # perf script
> ...
> a.out 395 59.338813: 11 cs: ffffffff81f4eeec schedul>
> a.out 395 59.339707: 12 cs: ffffffff81f4eeec schedul>
> a.out 395 59.340682: 13 cs: ffffffff81f4eeec schedul>
> a.out 395 59.341751: 13 cs: ffffffff81f4eeec schedul>
> a.out 395 59.342799: 12 cs: ffffffff81f4eeec schedul>
> a.out 395 59.343765: 11 cs: ffffffff81f4eeec schedul>
> a.out 395 59.344651: 11 cs: ffffffff81f4eeec schedul>
> a.out 395 59.345539: 12 cs: ffffffff81f4eeec schedul>
> a.out 395 59.346502: 13 cs: ffffffff81f4eeec schedul>
> ...
>
> test.c
>
> int main() {
> for (int i = 0; i < 20000; i++)
> usleep(10);
>
> return 0;
> }
>
> # time ./a.out
> real 0m1.583s
> user 0m0.040s
> sys 0m0.298s
>
> The above results were tested on x86-64 qemu with KVM enabled using
> test.c as test program. Ideally, we should have around 1500 samples,
> but the previous algorithm had only about 500, whereas the modified
> algorithm now has about 1400. Further more, the new version shows 1
> sample per 0.001s, while the previous one is 1 sample per 0.002s.This
> indicates that the new algorithm is more sensitive to small negative
> values compared to old algorithm.
>
> Fixes: bd2b5b12849a ("perf_counter: More aggressive frequency adjustment")
> Signed-off-by: Luo Gengkun <luogengkun@...weicloud.com>
> Reviewed-by: Adrian Hunter <adrian.hunter@...el.com>
You may want to Cc stable to back port it to the LTS kernel.
Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
Thanks,
Kan
> ---
> kernel/events/core.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c973e3c11e03..a9395bbfd4aa 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4092,7 +4092,11 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count, bo
> period = perf_calculate_period(event, nsec, count);
>
> delta = (s64)(period - hwc->sample_period);
> - delta = (delta + 7) / 8; /* low pass filter */
> + if (delta >= 0)
> + delta += 7;
> + else
> + delta -= 7;
> + delta /= 8; /* low pass filter */
>
> sample_period = hwc->sample_period + delta;
>
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