Messages in this thread | | | Date | Wed, 19 Jan 2022 11:59:44 +0000 | From | Vincent Donnefort <> | Subject | Re: [PATCH v2 2/7] sched/fair: Decay task PELT values during migration |
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> > > > > > This has several shortfalls: > > > - have a look at cfs_rq_clock_pelt() and rq_clock_pelt(). What you > > > name clock_pelt in your commit message and is used to update PELT and > > > saved in se->avg.last_update_time is : rq->clock_pelt - > > > rq->lost_idle_time - cfs_rq->throttled_clock_task_time > > > > That's why, the PELT "lag" is added onto se->avg.last_update_time. (see the last > > paragraph of the commit message) The estimator is just a time delta, that is > > added on top of the entity's last_update_time. I don't see any problem with the > > lost_idle_time here. > > lost_idle_time is updated before entering idle and after your > clock_pelt_lag has been updated. This means that the delta that you > are computing can be wrong > > I haven't look in details but similar problem probably happens for > throttled_clock_task_time > > > > > > - you are doing this whatever the state of the cpu : idle or not. But > > > the clock cycles are not accounted for in the same way in both cases. > > > > If the CPU is idle and clock_pelt == clock_task, the component A of the > > estimator would be 0 and we only would account for how outdated is the rq's > > clock, i.e. component B. > > And if cpu is not idle, you can't apply the diff between clk_pelt and clock_task > > > > > > - (B) doesn't seem to be accurate as you skip irq and steal time > > > accounting and you don't apply any scale invariance if the cpu is not > > > idle > > > > The missing irq and paravirt time is the reason why it is called "estimator". > > But maybe there's a chance of improving this part with a lockless version of > > rq->prev_irq_time and rq->prev_steal_time_rq? > > > > > - IIUC your explanation in the commit message above, the (A) period > > > seems to be a problem only when idle but you apply it unconditionally. > > > > If the CPU is idle (and clock_pelt == clock_task), only the B part would be > > worth something: > > > > A + B = [clock_task - clock_pelt] + [sched_clock_cpu() - clock] > > A B > > > > > If cpu is idle you can assume that clock_pelt should be equal to > > > clock_task but you can't if cpu is not idle otherwise your sync will > > > be inaccurate and defeat the primary goal of this patch. If your > > > problem with clock_pelt is that the pending idle time is not accounted > > > for when entering idle but only at the next update (update blocked > > > load or wakeup of a thread). This patch below should fix this and > > > remove your A. > > > > That would help slightly the current situation, but this part is already > > covered by the estimator. > > But the estimator, as you name it, is wrong beaus ethe A part can't be > applied unconditionally
Hum, it is used only in the !active migration. So we know the task was sleeping before that migration. As a consequence, the time we need to account is "sleeping" time from the task point of view, which is clock_pelt == clock_task (for __update_load_avg_blocked_se()). Otherwise, we would only decay with the "wallclock" idle time instead of the "scaled" one wouldn't we?
+-------------+-------------- | Task A | Task B ..... ^ ^ ^ | | migrate A | | | | | | | | | | |<----------->| | Wallclock Task A idle time |<---------------->| "Scaled" Task A idle time
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