Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 19 Jan 2022 10:54:36 +0100 | Subject | Re: [PATCH v2 2/7] sched/fair: Decay task PELT values during migration |
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On Tue, 18 Jan 2022 at 11:56, Vincent Donnefort <vincent.donnefort@arm.com> wrote: > > On Mon, Jan 17, 2022 at 06:31:25PM +0100, Vincent Guittot wrote: > > On Wed, 12 Jan 2022 at 17:14, Vincent Donnefort > > <vincent.donnefort@arm.com> wrote: > > > > > > Before being migrated to a new CPU, a task sees its PELT values > > > synchronized with rq last_update_time. Once done, that same task will also > > > have its sched_avg last_update_time reset. This means the time between > > > the migration and the last clock update (B) will not be accounted for in > > > util_avg and a discontinuity will appear. This issue is amplified by the > > > PELT clock scaling. If the clock hasn't been updated while the CPU is > > > idle, clock_pelt will not be aligned with clock_task and that time (A) > > > will be also lost. > > > > > > ---------|----- A -----|-----------|------- B -----|> > > > clock_pelt clock_task clock now > > > > > > This is especially problematic for asymmetric CPU capacity systems which > > > need stable util_avg signals for task placement and energy estimation. > > > > > > Ideally, this problem would be solved by updating the runqueue clocks > > > before the migration. But that would require taking the runqueue lock > > > which is quite expensive [1]. Instead estimate the missing time and update > > > the task util_avg with that value: > > > > > > A + B = clock_task - clock_pelt + sched_clock_cpu() - clock > > > > > > Neither clock_task, clock_pelt nor clock can be accessed without the > > > runqueue lock. The new runqueue clock_pelt_lag is therefore created and > > > encode those three values. > > > > > > clock_pelt_lag = clock - clock_task + clock_pelt > > > > > > And we can then write the missing time as follow: > > > > > > A + B = sched_clock_cpu() - clock_pelt_lag > > > > > > The B. part of the missing time is however an estimation that doesn't take > > > into account IRQ and Paravirt time. > > > > > > Now we have an estimation for A + B, we can create an estimator for the > > > PELT value at the time of the migration. We need for this purpose to > > > inject last_update_time which is a combination of both clock_pelt and > > > lost_idle_time. The latter is a time value which is completely lost form a > > > PELT point of view and must be ignored. And finally, we can write: > > > > > > rq_clock_pelt_estimator() = last_update_time + A + B > > > = last_update_time + > > > sched_clock_cpu() - clock_pelt_lag > > > > > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/ > > > > > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index 06cf7620839a..11c6aeef4583 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -618,6 +618,12 @@ struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf) > > > } > > > } > > > > > > +static void update_rq_clock_pelt_lag(struct rq *rq) > > > +{ > > > + u64_u32_store(rq->clock_pelt_lag, > > > + rq->clock - rq->clock_task + rq->clock_pelt); > > > > 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
> > > > > > > diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h > > index e06071bf3472..855877be4dd8 100644 > > --- a/kernel/sched/pelt.h > > +++ b/kernel/sched/pelt.h > > @@ -114,6 +114,7 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > > { > > u32 divider = ((LOAD_AVG_MAX - 1024) << SCHED_CAPACITY_SHIFT) > > - LOAD_AVG_MAX; > > u32 util_sum = rq->cfs.avg.util_sum; > > + u64 now = rq_clock_task(rq); > > util_sum += rq->avg_rt.util_sum; > > util_sum += rq->avg_dl.util_sum; > > > > @@ -127,7 +128,10 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > > * rq's clock_task. > > */ > > if (util_sum >= divider) > > - rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > > + rq->lost_idle_time += now - rq->clock_pelt; > > + > > + /* The rq is idle, we can sync to clock_task */ > > + rq->clock_pelt = now; > > } > > > > static inline u64 rq_clock_pelt(struct rq *rq) > > > > --- > > > > > > > +} > > > + > > > /* > > > * RQ-clock updating methods: > > > */ > > > @@ -674,6 +680,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta) > > > update_irq_load_avg(rq, irq_delta + steal); > > > #endif > > > update_rq_clock_pelt(rq, delta); > > > + update_rq_clock_pelt_lag(rq); > > > } > > > > > > void update_rq_clock(struct rq *rq) > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > index 99ea9540ece4..046d5397eb8a 100644 > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -6852,6 +6852,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) > > > > > > static void detach_entity_cfs_rq(struct sched_entity *se); > > > > > > +static u64 rq_clock_pelt_estimator(struct rq *rq, u64 last_update_time) > > > +{ > > > + u64 pelt_lag = sched_clock_cpu(cpu_of(rq)) - > > > + u64_u32_load(rq->clock_pelt_lag); > > > > Have you evaluated the impact of calling sched_clock_cpu(cpu_of(rq)) > > for a remote cpu ? especially with a huge number of migration and > > concurrent access from several cpus > > I have not, but I will have a look. > > > > > > + > > > + return last_update_time + pelt_lag; > > > +} > > > + > > > /* > > > * Called immediately before a task is migrated to a new CPU; task_cpu(p) and > > > * cfs_rq_of(p) references at time of call are still valid and identify the > > > @@ -6859,6 +6867,9 @@ static void detach_entity_cfs_rq(struct sched_entity *se); > > > */ > > > static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > > { > > > + struct sched_entity *se = &p->se; > > > + struct rq *rq = task_rq(p); > > > + > > > /* > > > * As blocked tasks retain absolute vruntime the migration needs to > > > * deal with this by subtracting the old and adding the new > > > @@ -6866,7 +6877,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > > * the task on the new runqueue. > > > */ > > > if (READ_ONCE(p->__state) == TASK_WAKING) { > > > - struct sched_entity *se = &p->se; > > > struct cfs_rq *cfs_rq = cfs_rq_of(se); > > > > > > se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > > > @@ -6877,26 +6887,32 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > > * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old' > > > * rq->lock and can modify state directly. > > > */ > > > - lockdep_assert_rq_held(task_rq(p)); > > > - detach_entity_cfs_rq(&p->se); > > > + lockdep_assert_rq_held(rq); > > > + detach_entity_cfs_rq(se); > > > > > > } else { > > > + u64 now; > > > + > > > + remove_entity_load_avg(se); > > > + > > > /* > > > - * We are supposed to update the task to "current" time, then > > > - * its up to date and ready to go to new CPU/cfs_rq. But we > > > - * have difficulty in getting what current time is, so simply > > > - * throw away the out-of-date time. This will result in the > > > - * wakee task is less decayed, but giving the wakee more load > > > - * sounds not bad. > > > + * Here, the task's PELT values have been updated according to > > > + * the current rq's clock. But if that clock hasn't been > > > + * updated in a while, a substantial idle time will be missed, > > > + * leading to an inflation after wake-up on the new rq. > > > + * > > > + * Estimate the PELT clock lag, and update sched_avg to ensure > > > + * PELT continuity after migration. > > > */ > > > - remove_entity_load_avg(&p->se); > > > + now = rq_clock_pelt_estimator(rq, se->avg.last_update_time); > > > + __update_load_avg_blocked_se(now, se); > > > } > > > > > > /* Tell new CPU we are migrated */ > > > - p->se.avg.last_update_time = 0; > > > + se->avg.last_update_time = 0; > > > > > > /* We have migrated, no longer consider this task hot */ > > > - p->se.exec_start = 0; > > > + se->exec_start = 0; > > > > > > update_scan_period(p, new_cpu); > > > } > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > > index f1a445efdc63..fdf2a9e54c0e 100644 > > > --- a/kernel/sched/sched.h > > > +++ b/kernel/sched/sched.h > > > @@ -1027,8 +1027,13 @@ struct rq { > > > /* Ensure that all clocks are in the same cache line */ > > > u64 clock_task ____cacheline_aligned; > > > u64 clock_pelt; > > > + u64 clock_pelt_lag; > > > unsigned long lost_idle_time; > > > > > > +#ifndef CONFIG_64BIT > > > + u64 clock_pelt_lag_copy; > > > +#endif > > > + > > > atomic_t nr_iowait; > > > > > > #ifdef CONFIG_SCHED_DEBUG > > > -- > > > 2.25.1 > > >
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