linux-kernel - Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

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Date:	Thu, 24 Feb 2011 19:40:23 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Vincent Guittot <vincent.guittot@...aro.org>
cc:	linux-kernel@...r.kernel.org, linux-hotplug@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, amit.kucheria@...aro.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH V5 2/2] tracing, perf : add cpu hotplug trace events

On Thu, 24 Feb 2011, Vincent Guittot wrote:

Please send full patch series not a single V5 2/2 which lacks any
references to 0/2 1/2.

> Please find below a new proposal for adding trace events for cpu hotplug:

Either it's a patch or a proposal. Darn, why think people that
proposal is such a important word? It's just useless. You don't have
to sell anything to your manager. You provide a patch which is judged
on it's technical merits and correctness. Nothing else.

> -the lock/unlock of cpu_add_remove_lock mutex is now outside the trace
> 
> The goal is to measure the latency of each part (kernel, architecture)
> and also to trace the cpu hotplug activity with other power events. I
> have tested these traces events on an arm platform.

This belongs into a cover mail [0/2] not into the patch itself

> Subject: [PATCH 2/2] add hotplug tracepoint

While your mail subject is correct, this is not.

If you would have sent a [0/2] cover mail with all the above blurb in
it then this extra subject line would be not needed at all.

> this patch adds new events for cpu hotplug tracing

Sentences start with an upper case letter. 

Also we already know that this is a patch. Where is the value of this
changelog? It does not tell more than the subject line.

>  * plug/unplug sequence

How surprising.

>  * core and architecture latency measurements

No it does not. It does not add latency measurements. It merily adds
tracepoints which allow you to compute the time spent in the various
steps of the hotplug state machine and the overall time.

>  #ifdef CONFIG_SMP
>  /* Serializes the updates to cpu_online_mask, cpu_present_mask */
>  static DEFINE_MUTEX(cpu_add_remove_lock);
> @@ -197,10 +200,13 @@ struct take_cpu_down_param {
>  static int __ref take_cpu_down(void *_param)
>  {
>  	struct take_cpu_down_param *param = _param;
> +	unsigned int cpu = (unsigned int)(param->hcpu);
>  	int err;
> 
>  	/* Ensure this CPU doesn't handle any more interrupts. */
> +	trace_cpu_hotplug_disable_start(cpu);
>  	err = __cpu_disable();
> +	trace_cpu_hotplug_disable_end(cpu);

How useful. What about recording the return code of __cpu_disable()?

>  	if (err < 0)
>  		return err;

> +	trace_cpu_hotplug_down_start(cpu);
> +

What's the point of this tracepoint _BEFORE_ the cpu_hotplug_disabled
check without recording cpu_hotplug_disabled ?

>  	if (cpu_hotplug_disabled) {
>  		err = -EBUSY;
>  		goto out;
> @@ -284,6 +294,8 @@ int __ref cpu_down(unsigned int cpu)
>  	err = _cpu_down(cpu, 0);
> 
>  out:
> +	trace_cpu_hotplug_down_end(cpu);

And this one is misplaced as well. It wants to be only called when we
actually called _cpu_down() and it wants to record the return code as
well.

> +
>  	cpu_maps_update_done();
>  	return err;
>  }
> @@ -310,7 +322,9 @@ static int __cpuinit _cpu_up(unsigned int cpu, int
> tasks_frozen)
>  	}
> 
>  	/* Arch-specific enabling code. */
> +	trace_cpu_hotplug_arch_up_start(cpu);
>  	ret = __cpu_up(cpu);
> +	trace_cpu_hotplug_arch_up_end(cpu);

See above.

>  	if (ret != 0)
>  		goto out_notify;
>  	BUG_ON(!cpu_online(cpu));
> @@ -369,6 +383,8 @@ int __cpuinit cpu_up(unsigned int cpu)
> 
>  	cpu_maps_update_begin();
> 
> +	trace_cpu_hotplug_up_start(cpu);
> +

Ditto

>  	if (cpu_hotplug_disabled) {
>  		err = -EBUSY;
>  		goto out;
> @@ -377,6 +393,8 @@ int __cpuinit cpu_up(unsigned int cpu)
>  	err = _cpu_up(cpu, 0);
> 
>  out:
> +	trace_cpu_hotplug_up_end(cpu);
> +

Sigh.

>  	cpu_maps_update_done();
>  	return err;

Thanks,

	tglx
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