Re: [PATCH] thermal: cpufreq_cooling: freq_qos_update_request() returns < 0 on error

[Lukasz Luba]

On 2/17/21 10:39 AM, Viresh Kumar wrote:
> On 17-02-21, 10:29, Lukasz Luba wrote:
> > On 2/17/21 5:48 AM, Viresh Kumar wrote:
> > > freq_qos_update_request() returns 1 if the effective constraint value
> > > has changed, 0 if the effective constraint value has not changed, or a
> > > negative error code on failures.
> > >
> > > The frequency constraints for CPUs can be set by different parts of the
> > > kernel. If the maximum frequency constraint set by other parts of the
> > > kernel are set at a lower value than the one corresponding to cooling
> > > state 0, then we will never be able to cool down the system as
> > > freq_qos_update_request() will keep on returning 0 and we will skip
> > > updating cpufreq_state and thermal pressure.
> >
> > To be precised, thermal pressure signal is not so important in this
> > mechanism and the 'cpufreq_state' has changed recently:
>
> Right, I wasn't concerned only about no thermal cooling, but both
> thermal cooling and pressure.
>
> > 236761f19a4f373354  thermal/drivers/cpufreq_cooling: Update cpufreq_state
> > only if state has changed
>
> This moved the assignment to a more logical place for me, i.e. not to
> do that on errors, just that the block in which it landed may not get
> called at all :(
>
> > > Fix that by doing the updates even in the case where
> > > freq_qos_update_request() returns 0, as we have effectively set the
> > > constraint to a new value even if the consolidated value of the
> > > actual constraint is unchanged because of external factors.
> > >
> > > Cc: v5.7+ <stable@xxxxxxxxxxxxxxx> # v5.7+
> > > Reported-by: Thara Gopinath <thara.gopinath@xxxxxxxxxx>
> > > Fixes: f12e4f66ab6a ("thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping")
> >
> > I'm not sure if that f12e4f is the root cause.
>
> Hmm, depends on how we define the problem :)
>
> If this was just about thermal-cooling not happening, then may be yes,
> but to me it is rather about mishandled return value of
> freq_qos_update_request() which has more than one side effects and so
> I went for the main commit.
>
> This is also important as f12e4f66ab6a got merged in 5.7 and 236761f19
> merged in 5.11 and this patch needs to get applied in stable kernels
> since 5.7 to fix it all.

'to fix it all' - I agree