Messages in this thread | | | Date | Wed, 19 May 2021 17:06:34 +0100 | From | Vincent Donnefort <> | Subject | Re: [PATCH] sched/fair: Fix util_est UTIL_AVG_UNCHANGED handling |
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On Fri, May 14, 2021 at 12:37:48PM +0200, Dietmar Eggemann wrote: > The util_est internal UTIL_AVG_UNCHANGED flag which is used to prevent > unnecessary util_est updates uses the LSB of util_est.enqueued. It is > exposed via _task_util_est() (and task_util_est()). > > Commit 92a801e5d5b7 ("sched/fair: Mask UTIL_AVG_UNCHANGED usages") > mentions that the LSB is lost for util_est resolution but > find_energy_efficient_cpu() checks if task_util_est() returns 0 to > return prev_cpu early. > > _task_util_est() returns the max value of util_est.ewma and > util_est.enqueued or'ed w/ UTIL_AVG_UNCHANGED. > So task_util_est() returning the max of task_util() and > _task_util_est() will never return 0 under the default > SCHED_FEAT(UTIL_EST, true). > > To fix this use the MSB of util_est.enqueued instead and keep the flag > util_est internal, i.e. don't export it via _task_util_est(). > > The maximal possible util_avg value for a task is 1024 so the MSB of > 'unsigned int util_est.enqueued' isn't used to store a util value. > > As a caveat the code behind the util_est_se trace point has to filter > UTIL_AVG_UNCHANGED to see the real util_est.enqueued value which should > be easy to do. > > This also fixes an issue report by Xuewen Yan that util_est_update() > only used UTIL_AVG_UNCHANGED for the subtrahend of the equation: > > last_enqueued_diff = ue.enqueued - (task_util() | UTIL_AVG_UNCHANGED) > > Fixes: b89997aa88f0b sched/pelt: Fix task util_est update filtering > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
I don't believe this made it through our integration branch testing, so I gave a try manually on my Hikey with LISA's UtilConvergence test. 20 iterations of that test passed without any problem.
> --- > kernel/sched/fair.c | 5 +++-- > kernel/sched/pelt.h | 13 +++++++------ > 2 files changed, 10 insertions(+), 8 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 161b92aa1c79..0150d440b0a2 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3856,7 +3856,7 @@ static inline unsigned long _task_util_est(struct task_struct *p) > { > struct util_est ue = READ_ONCE(p->se.avg.util_est); > > - return (max(ue.ewma, ue.enqueued) | UTIL_AVG_UNCHANGED); > + return max(ue.ewma, (ue.enqueued & ~UTIL_AVG_UNCHANGED)); > }
Shall we also update proc_sched_show_task() to avoid reading this flag there?
-- Vincent
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