Messages in this thread | | | Date | Thu, 1 Oct 2015 11:17:00 +0100 | From | Javi Merino <> | Subject | Re: [PATCH] thermal: avoid division by zero in power allocator |
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On Tue, Sep 29, 2015 at 09:33:30PM +0100, Andrew Morton wrote: > On Mon, 28 Sep 2015 23:28:34 +0200 Andrea Arcangeli <aarcange@redhat.com> wrote: > > > During boot I get a div by zero Oops regression starting in v4.3-rc3. > > > > ... > > > > --- a/drivers/thermal/power_allocator.c > > +++ b/drivers/thermal/power_allocator.c > > @@ -144,6 +144,16 @@ static void estimate_pid_constants(struct thermal_zone_device *tz, > > switch_on_temp = 0; > > > > temperature_threshold = control_temp - switch_on_temp; > > + /* > > + * estimate_pid_constants() tries to find appropriate default > > + * values for thermal zones that don't provide them. If a > > + * system integrator has configured a thermal zone with two > > + * passive trip points at the same temperature, that person > > + * hasn't put any effort to set up the thermal zone properly > > + * so just give up. > > + */ > > + if (!temperature_threshold) > > + return; > > > > if (!tz->tzp->k_po || force) > > tz->tzp->k_po = int_to_frac(sustainable_power) / > > a) Are we sure this won't leave tz->tzp fields uninitialized?
They will be all zeros. That's good enough.
> b) I'm not understanding that code at all. The "proportional" term > in a PID controller is supposed to be proportional to the (desired - > actual) difference (aka "the error"). > > But estimate_pid_constants() appears to be setting the > "proportional" term to be proportional to 1/error!
estimate_pid_constants() calculate the constants that you use in the PID algorithm. Say:
k_p * error + k_i * integral_of_error + k_d * diff_of_error
This code is calculating a reasonable k_p, k_i and k_d when they are not provided by the platform.
> Maybe a description of local `temperature_threshold' would help > clue me in.
The `error' in the above definition is:
target_temperature - current_temperature
whereas `temperature_threshold' is:
`target_temperature' - `switch_on_temperature'
`switch_on_temperature' is the temperature above which the thermal governor starts operating and throttling cpus (or whatever cooling device is configured).
The `switch_on_temperature' and `target_temperature' are defined using trip points. A platform that sets two trip points to the same temperature is not properly configured. With Andrea's patch we provide degraded behavior instead of crashing. I agree with that approach (hence my Reviewed-by, maybe it should be an Acked-by?).
Cheers, Javi
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