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Message-ID: <8977468f-26d1-1945-0d11-e56b10ff47c0@huawei.com>
Date: Thu, 14 Oct 2021 16:09:34 +0800
From: Yang Jihong <yangjihong1@...wei.com>
To: Steven Rostedt <rostedt@...dmis.org>
CC: <mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: save cmdline only when task does not exist in
savecmd for optimization
Hi Steven,
On 2021/10/14 11:02, Steven Rostedt wrote:
> On Mon, 11 Oct 2021 19:50:18 +0800
> Yang Jihong <yangjihong1@...wei.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@...wei.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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