Messages in this thread | | | Subject | Re: [PATCH] tracing: save cmdline only when task does not exist in savecmd for optimization | From | Yang Jihong <> | Date | Thu, 14 Oct 2021 16:09:34 +0800 |
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Hi Steven,
On 2021/10/14 11:02, Steven Rostedt wrote: > On Mon, 11 Oct 2021 19:50:18 +0800 > Yang Jihong <yangjihong1@huawei.com> wrote: > >> commit 85f726a35e504418 use strncpy instead of memcpy when copying comm, >> on ARM64 machine, this commit causes performance degradation. >> >> For the task that already exists in savecmd, it is unnecessary to call >> set_cmdline to execute strncpy once, run set_cmdline only if the task does >> not exist in savecmd. >> >> I have written an example (which is an extreme case) in which trace sched switch >> is invoked for 1000 times, as shown in the following: >> >> for (int i = 0; i < 1000; i++) { >> trace_sched_switch(true, current, current); >> } > > Well that's a pretty non realistic benchmark. > >> >> On ARM64 machine, compare the data before and after the optimization: >> +---------------------+------------------------------+------------------------+ >> | | Total number of instructions | Total number of cycles | >> +---------------------+------------------------------+------------------------+ >> | Before optimization | 1107367 | 658491 | >> +---------------------+------------------------------+------------------------+ >> | After optimization | 869367 | 520171 | >> +---------------------+------------------------------+------------------------+ >> As shown above, there is nearly 26% performance > > I'd prefer to see a more realistic benchmark. > >> >> Signed-off-by: Yang Jihong <yangjihong1@huawei.com> >> --- >> kernel/trace/trace.c | 7 +++++-- >> 1 file changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c >> index 7896d30d90f7..a795610a3b37 100644 >> --- a/kernel/trace/trace.c >> +++ b/kernel/trace/trace.c >> @@ -2427,8 +2427,11 @@ static int trace_save_cmdline(struct task_struct *tsk) >> savedcmd->cmdline_idx = idx; >> } >> >> - savedcmd->map_cmdline_to_pid[idx] = tsk->pid; >> - set_cmdline(idx, tsk->comm); >> + /* save cmdline only when task does not exist in savecmd */ >> + if (savedcmd->map_cmdline_to_pid[idx] != tsk->pid) { >> + savedcmd->map_cmdline_to_pid[idx] = tsk->pid; >> + set_cmdline(idx, tsk->comm); >> + } > > I'm not against adding this. Just for kicks I ran the following before > and after this patch: > > # trace-cmd start -e sched > # perf stat -r 100 hackbench 50 > > Before: > > Performance counter stats for '/work/c/hackbench 50' (100 runs): > > 6,261.26 msec task-clock # 6.126 CPUs utilized ( +- 0.12% ) > 93,519 context-switches # 14.936 K/sec ( +- 1.12% ) > 13,725 cpu-migrations # 2.192 K/sec ( +- 1.16% ) > 47,266 page-faults # 7.549 K/sec ( +- 0.54% ) > 22,911,885,026 cycles # 3.659 GHz ( +- 0.11% ) > 15,171,250,777 stalled-cycles-frontend # 66.22% frontend cycles idle ( +- 0.13% ) > 18,330,841,604 instructions # 0.80 insn per cycle > # 0.83 stalled cycles per insn ( +- 0.11% ) > 4,027,904,559 branches # 643.306 M/sec ( +- 0.11% ) > 31,327,782 branch-misses # 0.78% of all branches ( +- 0.20% ) > > 1.02201 +- 0.00158 seconds time elapsed ( +- 0.15% ) > After: > > Performance counter stats for '/work/c/hackbench 50' (100 runs): > > 6,216.47 msec task-clock # 6.124 CPUs utilized ( +- 0.10% ) > 93,311 context-switches # 15.010 K/sec ( +- 0.91% ) > 13,719 cpu-migrations # 2.207 K/sec ( +- 1.09% ) > 47,085 page-faults # 7.574 K/sec ( +- 0.49% ) > 22,746,703,318 cycles # 3.659 GHz ( +- 0.09% ) > 15,012,911,121 stalled-cycles-frontend # 66.00% frontend cycles idle ( +- 0.11% ) > 18,275,147,949 instructions # 0.80 insn per cycle > # 0.82 stalled cycles per insn ( +- 0.08% ) > 4,017,673,788 branches # 646.295 M/sec ( +- 0.08% ) > 31,313,459 branch-misses # 0.78% of all branches ( +- 0.17% ) > > 1.01506 +- 0.00150 seconds time elapsed ( +- 0.15% ) > > Really it's all in the noise, so adding this doesn't seem to hurt. > Thanks very much for benchmark test data. :) Indeed, the effect of this modification is not obvious in scenarios where tasks are repeatedly created, but only in scenarios where tasks are repeatedly scheduled between a limited number of tasks.
Thanks, Jihong > -- Steve > > > >> >> arch_spin_unlock(&trace_cmdline_lock); >> > > . >
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