Re: [PATCH v5 4/4] sched/fair: update util_est only on util_avg updates

[Patrick Bellasi]

The changelog is missing the below CCs. :(

Since that's a new patch in this series, I expect some feedbacks and
thus I'll add them on the next respin.

On 22-Feb 17:01, Patrick Bellasi wrote:
> The estimated utilization of a task is currently updated every time the
> task is dequeued. However, to keep overheads under control, PELT signals
> are effectively updated at maximum once every 1ms.
> 
> Thus, for really short running tasks, it can happen that their util_avg
> value has not been updates since their last enqueue.  If such tasks are
> also frequently running tasks (e.g. the kind of workload generated by
> hackbench) it can also happen that their util_avg is updated only every
> few activations.
> 
> This means that updating util_est at every dequeue potentially introduces
> not necessary overheads and it's also conceptually wrong if the util_avg
> signal has never been updated during a task activation.
> 
> Let's introduce a throttling mechanism on task's util_est updates
> to sync them with util_avg updates. To make the solution memory
> efficient, both in terms of space and load/store operations, we encode a
> synchronization flag into the LSB of util_est.enqueued.
> This makes util_est an even values only metric, which is still
> considered good enough for its purpose.
> The synchronization bit is (re)set by __update_load_avg_se() once the
> PELT signal of a task has been updated during its last activation.
> 
> Such a throttling mechanism allows to keep under control util_est
> overheads in the wakeup hot path, thus making it a suitable mechanism
> which can be enabled also on high-intensity workload systems.
> Thus, this now switches on by default the estimation utilization
> scheduler feature.
> 
> Suggested-by: Chris Redpath <chris.redpath@xxxxxxx>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx

[...]

-- 
#include <best/regards.h>

Patrick Bellasi