Re: [PATCH 2/2][RFC] PM-runtime: add tracepoints to cover all usage_count changes

[Chen Yu]

On Sun, Jun 07, 2020 at 06:55:35AM +0200, Michal Miroslaw wrote:
> 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.
>
Okay, will do in next version, thanks!

Chenyu