Messages in this thread | | | Date | Wed, 13 Oct 2021 23:02:01 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] tracing: save cmdline only when task does not exist in savecmd for optimization |
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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.
-- Steve
> > arch_spin_unlock(&trace_cmdline_lock); >
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