Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Tue, 19 Apr 2022 18:27:58 +0200 | From | Vincent Guittot <> | Subject | Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration |
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Le mardi 19 avril 2022 à 13:23:27 (+0100), Vincent Donnefort a écrit : > > > On 19/04/2022 11:08, Vincent Guittot wrote: > > On Tue, 12 Apr 2022 at 15:42, 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 cfs_rq last_update_lag is therefore created and > > > contains those three values when the last_update_time value for that very > > > same cfs_rq is updated. > > > > > > last_update_lag = clock - clock_task + clock_pelt > > > > > > And we can then write the missing time as follow: > > > > > > A + B = sched_clock_cpu() - last_update_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 from a > > > PELT point of view and must be ignored. And finally, we can write: > > > > > > now = last_update_time + A + B > > > = last_update_time + sched_clock_cpu() - last_update_lag > > > > > > This estimation has a cost, mostly due to sched_clock_cpu(). Limit the > > > usage to the case where the source CPU is idle as we know this is when the > > > clock is having the biggest risk of being outdated. > > > > > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/ > > > > > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
[...]
> > > > I'm worried that we will call this for each and every > > update_cfs_rq_load_avg() whereas the content will be used only when > > idle and not throttled. Can't we use these conditions to save values > > only when needed and limit the number of useless updates ? > > I don't think we can use idle here as a condition, once it is idle, it is > too late to get those clock values.
As an example, the patch below should work. It doesn't handle the throttled case yet and still has to make sure that rq->enter_idle and rq->clock_pelt_idle are coherent in regards to ILB that update blocked load.
--- kernel/sched/fair.c | 30 ++++++++++++++++++++++++++++++ kernel/sched/pelt.h | 21 ++++++++++++++------- kernel/sched/sched.h | 3 ++- 3 files changed, 46 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e6ecf530f19f..f00843f9dd01 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7005,6 +7005,35 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) static void detach_entity_cfs_rq(struct sched_entity *se); +#ifdef CONFIG_NO_HZ_COMMON +static inline void migrate_se_pelt_lag(struct sched_entity *se) +{ + u64 now; + struct cfs_rq *cfs_rq; + struct rq *rq; + bool is_idle; + + cfs_rq = cfs_rq_of(se); + rq = rq_of(cfs_rq); + + rcu_read_lock(); + is_idle = is_idle_task(rcu_dereference(rq->curr)); + rcu_read_unlock(); + + if (!is_idle) + return; + + /* TODO handle throttled cfs */ + /* TODO handle update ilb blocked load update */ + now = READ_ONCE(rq->clock_pelt_idle); + now += sched_clock_cpu(cpu_of(rq)) - READ_ONCE(rq->enter_idle); + + __update_load_avg_blocked_se(now, se); +} +#else +static void migrate_se_pelt_lag(struct sched_entity *se) {} +#endif + /* * 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 @@ -7056,6 +7085,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) * sounds not bad. */ remove_entity_load_avg(&p->se); + migrate_se_pelt_lag(&p->se); } /* Tell new CPU we are migrated */ diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h index c336f5f481bc..ece4423026e5 100644 --- a/kernel/sched/pelt.h +++ b/kernel/sched/pelt.h @@ -61,6 +61,14 @@ static inline void cfs_se_util_change(struct sched_avg *avg) WRITE_ONCE(avg->util_est.enqueued, enqueued); } +static inline u64 rq_clock_pelt(struct rq *rq) +{ + lockdep_assert_rq_held(rq); + assert_clock_updated(rq); + + return rq->clock_pelt - rq->lost_idle_time; +} + /* * The clock_pelt scales the time to reflect the effective amount of * computation done during the running delta time but then sync back to @@ -78,6 +86,8 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta) if (unlikely(is_idle_task(rq->curr))) { /* The rq is idle, we can sync to clock_task */ rq->clock_pelt = rq_clock_task(rq); + WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */ + WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */ return; } @@ -130,14 +140,11 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) */ if (util_sum >= divider) rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; -} -static inline u64 rq_clock_pelt(struct rq *rq) -{ - lockdep_assert_rq_held(rq); - assert_clock_updated(rq); - - return rq->clock_pelt - rq->lost_idle_time; + /* The rq is idle, we can sync with clock_task */ + rq->clock_pelt = rq_clock_task(rq); + WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */ + WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */ } #ifdef CONFIG_CFS_BANDWIDTH diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 6ab77b171656..108698446762 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1007,7 +1007,8 @@ struct rq { u64 clock_task ____cacheline_aligned; u64 clock_pelt; unsigned long lost_idle_time; - + u64 clock_pelt_idle; + u64 enter_idle; atomic_t nr_iowait; #ifdef CONFIG_SCHED_DEBUG -- 2.17.1 > > > > A quick test with hackbench on a 8 cpus system gives > > around 60k call for a duration 550 msec run a root level > > and 180k from a 3rd level cgroups > > If you think this is too much to have this store on this path, we could > though either: > > a. restrict the feature to when we know it is the most important: EAS? > > b. store the updated value only when "decayed" is non 0. > > [...]
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