linux-kernel - Re: [PATCH 2/2][RFC] PM-runtime: add tracepoints to cover all usage

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Date:   Sun, 7 Jun 2020 06:55:35 +0200
From:   Michal Miroslaw <mirq-linux@...e.qmqm.pl>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2][RFC] PM-runtime: add tracepoints to cover all
 usage_count changes

On Sat, Jun 06, 2020 at 03:14:59PM +0800, Chen Yu wrote:
> Hi,
> On Fri, Jun 05, 2020 at 09:33:11PM +0200, Michal Miroslaw wrote:
> > On Sat, Jun 06, 2020 at 03:05:52AM +0800, Chen Yu wrote:
> > > Commit d229290689ae ("PM-runtime: add tracepoints for usage_count changes")
> > > has added some tracepoints to monitor the change of runtime usage, and
> > > there is something to improve:
> > > 1. There are some places that adjust the usage count have not
> > >    been traced yet. For example, pm_runtime_get_noresume() and
> > >    pm_runtime_put_noidle()
> > > 2. The change of the usage count will not be tracked if decreased
> > >    from 1 to 0.
> > [...]
> > > @@ -1448,16 +1453,17 @@ EXPORT_SYMBOL_GPL(pm_runtime_forbid);
> > >   */
> > >  void pm_runtime_allow(struct device *dev)
> > >  {
> > > +	bool is_zero;
> > > +
> > >  	spin_lock_irq(&dev->power.lock);
> > >  	if (dev->power.runtime_auto)
> > >  		goto out;
> > >  
> > >  	dev->power.runtime_auto = true;
> > > -	if (atomic_dec_and_test(&dev->power.usage_count))
> > > +	is_zero = atomic_dec_and_test(&dev->power.usage_count);
> > > +	trace_rpm_usage_rcuidle(dev, RPM_AUTO | RPM_ASYNC);
> > > +	if (is_zero)
> > >  		rpm_idle(dev, RPM_AUTO | RPM_ASYNC);
> > > -	else
> > > -		trace_rpm_usage_rcuidle(dev, RPM_AUTO | RPM_ASYNC);
> > > -
> > [...]
> > 
> > IIRC, rpm_idle() has a tracepoint already.
> > 
> Yes, this is what I concerned previously. If someone
> want to track the change of usage_count, then he might
> have to enable both trace rpm_usage and rpm_idle so
> as to track the moment when the counter drops from 1 to
> 0. It might be more consistent if we only enable
> trace rpm_usage to track the whole process.
> > > @@ -1523,9 +1529,8 @@ static void update_autosuspend(struct device *dev, int old_delay, int old_use)
> > >  		/* If it used to be allowed then prevent it. */
> > >  		if (!old_use || old_delay >= 0) {
> > >  			atomic_inc(&dev->power.usage_count);
> > > -			rpm_resume(dev, 0);
> > > -		} else {
> > >  			trace_rpm_usage_rcuidle(dev, 0);
> > > +			rpm_resume(dev, 0);
> > >  		}
> > >  	}
> > [...]
> > 
> > This actually changes logic, so it doesn't match the patch description.
> > 
> This patch intends to adjust the logic to be consistent with
> the change of usage_counter, that is to say, only after the
> counter has been possibly modified, we record it. In current
> logic above, it tracks the usage count where the latter does
> not change.

I see now what you intended. I think it would be nice to put the idea
(that all usage changes be shown using rpm_usage even if included in
other trace points) into the commit message. Otherwise, looks ok.

Best Regards
Michał Mirosław