Re: [PATCH v2 3/6] treewide: Replace memcpy(..., current->comm) with strscpy()

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 24 May 2026 19:38:53 -0300
André Almeida <andrealmeid@igalia.com> wrote:

> In order to increase the size of current->comm[] and to avoid breaking any
> existing code, replace memcpy() with strscpy(). The later function makes
> sure that the copy is NUL terminated. This is crucial given that the
> source buffer might be larger than the destination buffer and could
> truncate the NUL character out of it.
> 
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> ---
> Changes from v2:
>  - New patch, dropped strtostr() from last version
> ---
>  include/linux/coredump.h        |  2 +-
>  include/linux/tracepoint.h      |  4 ++--
>  include/trace/events/block.h    | 10 +++++-----
>  include/trace/events/coredump.h |  2 +-
>  include/trace/events/f2fs.h     |  4 ++--
>  include/trace/events/oom.h      |  2 +-
>  include/trace/events/osnoise.h  |  2 +-
>  include/trace/events/sched.h    | 10 +++++-----
>  include/trace/events/signal.h   |  2 +-
>  include/trace/events/task.h     |  4 ++--
>  kernel/printk/nbcon.c           |  2 +-
>  kernel/printk/printk.c          |  2 +-
>  12 files changed, 23 insertions(+), 23 deletions(-)
> 

So I was curious to what impact this would have on tracing. I decided to
run the following:

    perf stat -r 100 ./hackbench 50

To see how it affects things. Hackbench is a bit of a microbenchmark but it
stresses the scheduler and thus, scheduler trace events.

I first ran the above and put the output into "stat.baseline", then I enabled
all scheduler trace events:

   trace-cmd start -e sched

and ran it again and put the output into "stat.before".

I applied the patch and ran it again before enabling tracing (just to see
the variance) and put that into "stat.baseline2". I then enabled tracing
and ran it again and put the output into "stat.after".

Here's the results:

stat.baseline:

 Performance counter stats for '/work/c/hackbench 50' (100 runs):

            53,165      context-switches                 #  11002.2 cs/sec  cs_per_second       ( +-  1.33% )
             8,010      cpu-migrations                   #   1657.6 migrations/sec  migrations_per_second  ( +-  0.90% )
            53,936      page-faults                      #  11161.7 faults/sec  page_faults_per_second  ( +-  0.50% )
          4,832.24 msec task-clock                       #      6.0 CPUs  CPUs_utilized         ( +-  0.12% )
        18,787,710      branch-misses                    #      1.2 %  branch_miss_rate         ( +-  0.17% )  (38.88%)
     1,452,653,496      branches                         #    300.6 M/sec  branch_frequency     ( +-  0.14% )  (61.55%)
    15,607,564,080      cpu-cycles                       #      3.2 GHz  cycles_frequency       ( +-  0.15% )  (56.21%)
     7,648,608,518      instructions                     #      0.5 instructions  insn_per_cycle  ( +-  0.11% )  (55.82%)
    12,025,223,911      stalled-cycles-frontend          #     0.77 frontend_cycles_idle        ( +-  0.14% )  (56.26%)

       0.808204663 +- 0.001059873 seconds time elapsed  ( +-  0.13% )

stat.before:

 Performance counter stats for '/work/c/hackbench 50' (100 runs):

            54,722      context-switches                 #  11041.0 cs/sec  cs_per_second       ( +-  1.35% )
             8,170      cpu-migrations                   #   1648.4 migrations/sec  migrations_per_second  ( +-  1.08% )
            54,295      page-faults                      #  10954.8 faults/sec  page_faults_per_second  ( +-  0.53% )
          4,956.27 msec task-clock                       #      6.0 CPUs  CPUs_utilized         ( +-  0.14% )
        19,304,657      branch-misses                    #      1.2 %  branch_miss_rate         ( +-  0.20% )  (37.27%)
     1,497,794,368      branches                         #    302.2 M/sec  branch_frequency     ( +-  0.17% )  (60.74%)
    16,037,658,236      cpu-cycles                       #      3.2 GHz  cycles_frequency       ( +-  0.16% )  (57.72%)
     7,875,024,533      instructions                     #      0.5 instructions  insn_per_cycle  ( +-  0.13% )  (57.83%)
    12,344,722,147      stalled-cycles-frontend          #     0.77 frontend_cycles_idle        ( +-  0.17% )  (55.77%)

       0.827636161 +- 0.001027531 seconds time elapsed  ( +-  0.12% )


stat.baseline2:

 Performance counter stats for '/work/c/hackbench 50' (100 runs):

            52,590      context-switches                 #  10837.7 cs/sec  cs_per_second       ( +-  1.18% )
             7,958      cpu-migrations                   #   1640.0 migrations/sec  migrations_per_second  ( +-  0.99% )
            53,819      page-faults                      #  11090.9 faults/sec  page_faults_per_second  ( +-  0.48% )
          4,852.52 msec task-clock                       #      6.0 CPUs  CPUs_utilized         ( +-  0.11% )
        18,933,395      branch-misses                    #      1.2 %  branch_miss_rate         ( +-  0.18% )  (37.13%)
     1,451,361,950      branches                         #    299.1 M/sec  branch_frequency     ( +-  0.13% )  (60.09%)
    15,683,586,735      cpu-cycles                       #      3.2 GHz  cycles_frequency       ( +-  0.13% )  (56.05%)
     7,628,894,710      instructions                     #      0.5 instructions  insn_per_cycle  ( +-  0.10% )  (57.22%)
    12,063,750,082      stalled-cycles-frontend          #     0.77 frontend_cycles_idle        ( +-  0.14% )  (57.11%)

       0.811536383 +- 0.001337259 seconds time elapsed  ( +-  0.16% )

stat.after:

 Performance counter stats for '/work/c/hackbench 50' (100 runs):

            53,799      context-switches                 #  10743.3 cs/sec  cs_per_second       ( +-  1.35% )
             8,095      cpu-migrations                   #   1616.5 migrations/sec  migrations_per_second  ( +-  0.86% )
            54,330      page-faults                      #  10849.4 faults/sec  page_faults_per_second  ( +-  0.55% )
          5,007.67 msec task-clock                       #      6.0 CPUs  CPUs_utilized         ( +-  0.13% )
        19,444,339      branch-misses                    #      1.2 %  branch_miss_rate         ( +-  0.21% )  (38.04%)
     1,504,382,421      branches                         #    300.4 M/sec  branch_frequency     ( +-  0.17% )  (60.42%)
    16,225,153,060      cpu-cycles                       #      3.2 GHz  cycles_frequency       ( +-  0.16% )  (56.19%)
     7,889,645,005      instructions                     #      0.5 instructions  insn_per_cycle  ( +-  0.16% )  (56.30%)
    12,488,115,947      stalled-cycles-frontend          #     0.77 frontend_cycles_idle        ( +-  0.16% )  (55.55%)

       0.835123855 +- 0.001015781 seconds time elapsed  ( +-  0.12% )


Looking at the difference between cpu-cycles of baseline and baseline2, we have:

  15,607,564,080 vs 15,683,586,735 where it went up by 0.4% (in the noise).

But when enabling tracing, we have between before and after:

  16,037,658,236 vs 16,225,153,060 which is 1.1%. May be low but not insignificant.

Where tracing enabled slowed the code down by 2.7% (16,037,658,236 vs 15,607,564,080)
having another 1% is quite an impact!

As tracing now slows it down by 3.9% which is a significant increase from 2.7%

I really rather keep memcpy() here.

-- Steve