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Message-ID: <04f7f660-29cd-4728-bf12-26e00e6f14ed@huawei.com>
Date: Sun, 31 Mar 2024 14:54:05 +0800
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Subject: Re: [PATCH v2] perf/core: Fix small negative period being ignored
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To: <peterz@infradead.org>
CC: <mingo@redhat.com>, <acme@kernel.org>, <namhyung@kernel.org>,
	<mark.rutland@arm.com>, <alexander.shishkin@linux.intel.com>,
	<jolsa@kernel.org>, <irogers@google.com>, <adrian.hunter@intel.com>,
	<linux-perf-users@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <20240312193801.520641-1-luogengkun2@huawei.com>
From: Luo Gengkun <luogengkun2@huawei.com>
In-Reply-To: <20240312193801.520641-1-luogengkun2@huawei.com>
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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@huawei.com>
> Reviewed-by: Adrian Hunter <adrian.hunter@intel.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: