linux-kernel - Re: [PATCH v2] tracing: shrink max latency ringbuffer if unnecessary

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Message-ID: <AANLkTinKPVPAMT2tsZ1it90K4Qu601jr12ulHZFOfFEy@mail.gmail.com>
Date:	Thu, 1 Jul 2010 20:48:59 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	rostedt@...dmis.org, LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v2] tracing: shrink max latency ringbuffer if unnecessary

2010/7/1 Lai Jiangshan <laijs@...fujitsu.com>:
> KOSAKI Motohiro wrote:
>>
>> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>> ---
>>  kernel/trace/trace.c              |   39 ++++++++++++++++++++++++++++++------
>>  kernel/trace/trace.h              |    1 +
>>  kernel/trace/trace_irqsoff.c      |    3 ++
>>  kernel/trace/trace_sched_wakeup.c |    2 +
>>  4 files changed, 38 insertions(+), 7 deletions(-)
>>
>
> Reviewed-by: Lai Jiangshan <laijs@...fujitsu.com
>
>
>> -
>> +     if (current_trace && current_trace->use_max_tr) {
>> +             /*
>> +              * We don't free the ring buffer. instead, resize it because
>> +              * The max_tr ring buffer has some state (e.g. ring->clock) and
>> +              * we want preserve it.
>> +              */
>> +             ring_buffer_resize(max_tr.buffer, 1);
>> +             max_tr.entries = 1;
>> +     }
>>       destroy_trace_option_files(topts);
>>
>>       current_trace = t;
>>
>>       topts = create_trace_option_files(current_trace);
>
> I think we can skip the two resize when current_trace->use_max_tr==1 && t->use_max_tr==1

Yup. but I don't think it's worthful because it's rarely operation.


>
>> +     if (current_trace->use_max_tr) {
>> +             ret = ring_buffer_resize(max_tr.buffer, global_trace.entries);
>> +             if (ret < 0)
>> +                     goto out;
>> +             max_tr.entries = global_trace.entries;
>> +     }
>>
>>       if (t->init) {
>>               ret = tracer_init(t, tr);
>
> Does we need to shrink it when tracer_init() fails?
> Although tracer_init() hardly fails, and there is no bad effect even we don't shrink it.

Nope. brief code of tracing_set_tracer() is here

========================================
        if (current_trace && current_trace->reset)
                current_trace->reset(tr);

        destroy_trace_option_files(topts);

        current_trace = t;

        topts = create_trace_option_files(current_trace);

        if (t->init) {
                ret = tracer_init(t, tr);
                if (ret)
                        goto out;
        }
========================================

That's mean, if t->init fail, we can't rollback old tracer. so your
suggested micro optimization
doesn't makes observable improvement, I think.

Thanks.
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