Messages in this thread | | | Subject | Re: [PATCH v2 1/2] sched/fair: Take thermal pressure into account while estimating energy | From | Lukasz Luba <> | Date | Thu, 10 Jun 2021 13:30:00 +0100 |
| |
On 6/10/21 1:19 PM, Vincent Guittot wrote: > On Thu, 10 Jun 2021 at 12:37, Lukasz Luba <lukasz.luba@arm.com> wrote: >> >> >> >> On 6/10/21 11:07 AM, Dietmar Eggemann wrote: >>> On 10/06/2021 11:04, Lukasz Luba wrote: >>>> >> >> [snip] >> >>>> Not always, it depends on thermal governor decision, workload and >>>> 'power actors' (in IPA naming convention). Then it depends when and how >>>> hard you clamp the CPUs. They (CPUs) don't have to be always >>>> overutilized, they might be even 50-70% utilized but the GPU reduced >>>> power budget by 2 Watts, so CPUs left with only 1W. Which is still OK >>>> for the CPUs, since they are only 'feeding' the GPU with new 'jobs'. >>> >>> All this pretty much confines the usefulness of you proposed change. A >>> precise description of it with the patches is necessary to allow people >>> to start from there while exploring your patches. >> >> OK, I see your point. >> >> [snip] >> >>>> True, I hope this description above would help to understand the >>>> scenario. >>> >>> This description belongs in the patch header. The scenario in which your >>> functionality would improve things has to be clear. >>> I'm sure that not everybody looking at this patches is immediately aware >>> on how IPA setups work and which specific setup you have in mind here. >> >> Agree. I will add this description into the patch header for v3. >> >> [snip] >> >>>> >>>> Yes, this code implementation tries to address those issues. >>> >>> The point I was making here is: why using the PELT signal >>> thermal_load_avg() and not per_cpu(thermal_pressure, cpu) directly, >>> given the fact that the latter perfectly represents the frequency clamping? >>> >> >> Good question. I wanted to be aligned with other parts in the fair.c >> like cpu_capacity() and all it's users. The CPU capacity is reduced by >> RT, DL, IRQ and thermal load avg, not the 'raw' value from the >> per-cpu variable. >> >> TBH I cannot recall what was the argument back then >> when thermal pressure geometric series was introduced. >> Maybe to have a better control how fast it raises and decays >> so other mechanisms in the scheduler will see the change in thermal >> as not so sharp... (?) >> >> >> Vincent do you remember the motivation to have geometric series >> in thermal pressure and not use just the 'raw' value from per-cpu? > > In order to have thermal pressure synced with other metrics used by > the scheduler like util/rt/dl_avg. As an example, when thermal > pressure will decrease because thermal capping is removed, the > utilization will increase at the same pace as thermal will decrease > and it will not create some fake spare cycle. util_avg is the average > expected utilization of the cpu, thermal pressure reflects the average > stolen capacity for the same averaging time scale but this can be the > result of a toggle between several OPP > > Using current capping is quite volatile to make a decision as it might > have changed by the time you apply your decision. >
So for this scenario, where we want to just align EAS with SchedUtil frequency decision, which is instantaneous and has 'raw' value of capping from policy->max, shouldn't we use:
thermal_pressure = arch_scale_thermal_pressure(cpu_id)
instead of geometric series thermal_pressure signal?
This would avoid the hassling with idle CPUs and not updated thermal signal.
| |