linux-kernel - [tip: perf/core] perf/core: Fix small negative period being ignored

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <172554860849.2215.13532771799874013503.tip-bot2@tip-bot2>
Date: Thu, 05 Sep 2024 15:03:28 -0000
From: "tip-bot2 for Luo Gengkun" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Luo Gengkun <luogengkun@...weicloud.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, stable@...r.kernel.org, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf/core: Fix small negative period being ignored

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     62c0b1061593d7012292f781f11145b2d46f43ab
Gitweb:        https://git.kernel.org/tip/62c0b1061593d7012292f781f11145b2d46f43ab
Author:        Luo Gengkun <luogengkun@...weicloud.com>
AuthorDate:    Sat, 31 Aug 2024 07:43:15 
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 05 Sep 2024 16:56:13 +02:00

perf/core: Fix small negative period being ignored

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Adrian Hunter <adrian.hunter@...el.com>
Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20240831074316.2106159-2-luogengkun@huaweicloud.com
---
 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 4acec97..67e115d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4183,7 +4183,11 @@ 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;