5.4-stable review patch. If anyone has any objections, please let me know.
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From: Luo Gengkun luogengkun@huaweicloud.com
commit 62c0b1061593d7012292f781f11145b2d46f43ab upstream.
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 Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Adrian Hunter adrian.hunter@intel.com Reviewed-by: Kan Liang kan.liang@linux.intel.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20240831074316.2106159-2-luogengkun@huaweicloud.co... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/events/core.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
--- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -3714,7 +3714,11 @@ static void perf_adjust_period(struct pe 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;