Re: [PATCH v4 1/2] perf/core: Fix small negative period being ignored

["Liang, Kan" <kan.liang@linux.intel.com>]

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@huaweicloud.com>
> Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>

You may want to Cc stable to back port it to the LTS kernel.

Reviewed-by: Kan Liang <kan.liang@linux.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;
>