linux-kernel - Re: [PATCH 1/2] cpu/hotplug: Add cpuhp

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Message-ID: <87tuvij6da.fsf@nanos.tec.linutronix.de>
Date:   Mon, 28 Sep 2020 16:58:09 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Prasad Sodagudi <psodagud@...eaurora.org>, rostedt@...dmis.org,
        qais.yousef@....com, peterz@...radead.org, mingo@...nel.org,
        cai@....pw, tyhicks@...onical.com, arnd@...db.de
Cc:     rameezmustafa@...eaurora.org, linux-kernel@...r.kernel.org,
        Prasad Sodagudi <psodagud@...eaurora.org>
Subject: Re: [PATCH 1/2] cpu/hotplug: Add cpuhp_latency trace event

On Wed, Sep 23 2020 at 16:37, Prasad Sodagudi wrote:
> Add ftrace event trace_cpuhp_latency to track cpu
> hotplug latency. It helps to track the hotplug latency
> impact by firmware changes and kernel cpu hotplug callbacks.

Why?

And even if that makes sense, the implementation makes absolutely no
sense at all.

      trace_cpuhp_callback_enter();
      callback();
      trace_cpuhp_callback_exit();

No point in all this start time and conditional sched_clock() muck.

But then again, hotplug is slow by definition and nobody cares.

Thanks,

        tglx