Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2 9/9] sched/fair: Consider capacity inversion in util_fits_cpu() | Date | Mon, 07 Nov 2022 18:58:23 +0000 |
| |
On 05/11/22 20:41, Qais Yousef wrote: > On 11/04/22 17:35, Valentin Schneider wrote: >> On 04/08/22 15:36, Qais Yousef wrote: >> > We do consider thermal pressure in util_fits_cpu() for uclamp_min only. >> > With the exception of the biggest cores which by definition are the max >> > performance point of the system and all tasks by definition should fit. >> > >> > Even under thermal pressure, the capacity of the biggest CPU is the >> > highest in the system and should still fit every task. Except when it >> > reaches capacity inversion point, then this is no longer true. >> > >> > We can handle this by using the inverted capacity as capacity_orig in >> > util_fits_cpu(). Which not only addresses the problem above, but also >> > ensure uclamp_max now considers the inverted capacity. Force fitting >> > a task when a CPU is in this adverse state will contribute to making the >> > thermal throttling last longer. >> > >> > Signed-off-by: Qais Yousef <qais.yousef@arm.com> >> > --- >> > kernel/sched/fair.c | 14 +++++++++----- >> > 1 file changed, 9 insertions(+), 5 deletions(-) >> > >> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> > index cb32dc9a057f..77ae343e32a3 100644 >> > --- a/kernel/sched/fair.c >> > +++ b/kernel/sched/fair.c >> > @@ -4293,12 +4293,16 @@ static inline int util_fits_cpu(unsigned long util, >> > * For uclamp_max, we can tolerate a drop in performance level as the >> > * goal is to cap the task. So it's okay if it's getting less. >> > * >> > - * In case of capacity inversion, which is not handled yet, we should >> > - * honour the inverted capacity for both uclamp_min and uclamp_max all >> > - * the time. >> > + * In case of capacity inversion we should honour the inverted capacity >> > + * for both uclamp_min and uclamp_max all the time. >> > */ >> > - capacity_orig = capacity_orig_of(cpu); >> > - capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu); >> > + capacity_orig = cpu_in_capacity_inversion(cpu); >> > + if (capacity_orig) { >> > + capacity_orig_thermal = capacity_orig; >> > + } else { >> > + capacity_orig = capacity_orig_of(cpu); >> > + capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu); >> > + } >> > >> >> IIUC the rq->cpu_capacity_inverted computation in update_cpu_capacity() can be >> summarised as: >> >> - If there is a PD with equal cap_orig, but higher effective (orig - thermal) >> capacity >> OR >> there is a PD with pd_cap_orig > cpu_effective_cap: >> rq->cpu_capacity_inverted = capacity_orig - thermal_load_avg(rq) >> >> - Else: >> rq->cpu_capacity_inverted = 0 >> >> Then, the code above uses either rq->cpu_capacity_inverted if it is >> non-zero, otherwise: >> >> capacity_orig - arch_scale_thermal_pressure(cpu); >> >> Why use average thermal pressure in one case, and use instantaneous >> thermal pressure in the other? > > There was a big debate on [1] about using avg vs instantaneous. >
Interesting thread, thanks for the link!
> I used avg for detecting inversion to be consistent with using average in in > scale_rt_capacity(). I didn't want the inversion state to be flipping too > quickly too. > > I used the instantaneous in the other check based on that discussion. It seemed > using the average is hurtful when for example the medium drops an OPP and by > not reacting quickly at wake up we lose the chance to place it on a big; which > if my memory didn't fail me is what Xuewen was seeing. >
OK So IIUC by using the inst. pressure you start excluding CPUs sooner, and with the avg pressure you keep those CPUs out (if the pressure remained long enough).
> [1] https://lore.kernel.org/lkml/24631a27-42d9-229f-d9b0-040ac993b749@arm.com/ >
>> >> Can't we get rid of rq->cpu_capacity_inverted and replace this whole thing >> with an unconditional >> >> capacity_orig_thermal = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu)); >> >> ? > > I can't see how we end up with equivalent behavior then. Or address the > concerns raised by Xuewen and Lukasz on the RT thread in regards to avg vs > instantaneous. > > Specifically, if we don't use the new rq->cpu_capacity_inverted we can't handle > the case where the task is requesting to run at maximum performance but a small > drop in thermal pressure means it won't fit anywhere. That PD is the best fit > until it hits an inversion. > > Originally I wanted to defer handling thermal pressure into a different series. > But Vincent thought it's better to handle it now. We want more data points from > more systems tbh. But I think what we have now is still a good improvement over > what we had before. > > Lukasz had a patch [2] which could allow making thermal_load_avg() more > acceptable for systems that care about faster response times. > > [2] https://lore.kernel.org/lkml/20220429091245.12423-1-lukasz.luba@arm.com/ > > > Thanks > > -- > Qais Yousef
| |