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Message-ID: <f1bba2b6-e672-4450-85f3-34ae9d9e18b2@huawei.com>
Date: Wed, 10 Apr 2024 17:35:06 +0800
From: Luo Gengkun <luogengkun2@...wei.com>
To: <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>,
<luogengkun2@...wei.com>
Subject: Re: [PATCH v2] perf/core: Fix small negative period being ignored
Hi Adrian, can you tell me what to do next? Is there anything that needs
to be changed?
Looking forward to your reply.
On 2024/3/31 14:54, Luo Gengkun wrote:
> Ping.
>
> On 2024/3/13 3:38, 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")
>>
>> Link:https://lore.kernel.org/all/7919005d-ca26-4cae-8c1c-4adea63704ce@huawei.com/
>> Signed-off-by: Luo Gengkun <luogengkun2@...wei.com>
>> Reviewed-by: Adrian Hunter <adrian.hunter@...el.com>
>>
>> ---
>> v1 -> v2: Fix incorrected time diff in tick adjust period and review
>>
>> It seems that the Timer Interrupts is not coming every TICK_NSEC when
>> system is idle. So the final period that we calculate will be bigger
>> than expected because we pass an incorrected nsec to perf_adjust_period.
>> As shown below, the unexcepted period is come from
>> perf_adjust_freq_unthr_context.
>> Moreover, we cannot re-adjust the period using
>> __perf_event_account_interrupt
>> because the overflow time is larger than 2 * TICK_NSEC. To fix this
>> problem, we can calculate the interval of Timer Interrupts using
>> perf_clock.
>>
>> # taskset --cpu 0 perf record -F 1000 -e cs -- taskset --cpu 1 ./test
>> [ perf record: Woken up 1 times to write data ]
>> [ perf record: Captured and wrote 0.010 MB perf.data (204 samples) ]
>>
>> # perf script
>> ...
>> test 865 265.377846: 16 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.378900: 15 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.379845: 14 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.380770: 14 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.381647: 15 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.382638: 16 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.383647: 16 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.384704: 15 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.385649: 14 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.386578: 152 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.396383: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.406183: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.415839: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.425445: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.435052: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.444708: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.454314: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.463970: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> test 865 265.473577: 154 cs: ffffffff832e927b
>> schedule+0x2b
>> ...
>> ---
>> ---
>> include/linux/perf_event.h | 1 +
>> kernel/events/core.c | 22 ++++++++++++++++++----
>> 2 files changed, 19 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index afb028c54f33..2708f1d0692c 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -265,6 +265,7 @@ struct hw_perf_event {
>> * State for freq target events, see __perf_event_overflow() and
>> * perf_adjust_freq_unthr_context().
>> */
>> + u64 freq_tick_stamp;
>> u64 freq_time_stamp;
>> u64 freq_count_stamp;
>> #endif
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 683dc086ef10..3f58d3803237 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -4078,7 +4078,11 @@ 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;
>> @@ -4108,7 +4112,7 @@ perf_adjust_freq_unthr_context(struct
>> perf_event_context *ctx, bool unthrottle)
>> {
>> struct perf_event *event;
>> struct hw_perf_event *hwc;
>> - u64 now, period = TICK_NSEC;
>> + u64 now, period, tick_stamp;
>> s64 delta;
>> /*
>> @@ -4147,6 +4151,10 @@ perf_adjust_freq_unthr_context(struct
>> perf_event_context *ctx, bool unthrottle)
>> */
>> event->pmu->stop(event, PERF_EF_UPDATE);
>> + tick_stamp = perf_clock();
>> + period = tick_stamp - hwc->freq_tick_stamp;
>> + hwc->freq_tick_stamp = tick_stamp;
>> +
>> now = local64_read(&event->count);
>> delta = now - hwc->freq_count_stamp;
>> hwc->freq_count_stamp = now;
>> @@ -4158,8 +4166,14 @@ perf_adjust_freq_unthr_context(struct
>> perf_event_context *ctx, bool unthrottle)
>> * to perf_adjust_period() to avoid stopping it
>> * twice.
>> */
>> - if (delta > 0)
>> - perf_adjust_period(event, period, delta, false);
>> + if (delta > 0) {
>> + /*
>> + * we skip first tick adjust period
>> + */
>> + if (likely(period != tick_stamp && period < 2*TICK_NSEC)) {
>> + perf_adjust_period(event, period, delta, false);
>> + }
>> + }
>> event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
>> next:
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