Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2] sched/fair: prefer prev cpu in asymmetric wakeup path | Date | Thu, 29 Oct 2020 11:16:43 +0000 |
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Hi Vincent,
On 28/10/20 17:44, Vincent Guittot wrote: > During fast wakeup path, scheduler always check whether local or prev cpus > are good candidates for the task before looking for other cpus in the > domain. With > commit b7a331615d25 ("sched/fair: Add asymmetric CPU capacity wakeup scan") > the heterogenous system gains a dedicated path but doesn't try to reuse > prev cpu whenever possible. If the previous cpu is idle and belong to the > LLC domain, we should check it 1st before looking for another cpu because > it stays one of the best candidate and this also stabilizes task placement > on the system. > > This change aligns asymmetric path behavior with symmetric one and reduces > cases where the task migrates across all cpus of the sd_asym_cpucapacity > domains at wakeup. > > This change does not impact normal EAS mode but only the overloaded case or > when EAS is not used. > > - On hikey960 with performance governor (EAS disable) > > ./perf bench sched pipe -T -l 50000 > mainline w/ patch > # migrations 999364 0 > ops/sec 149313(+/-0.28%) 182587(+/- 0.40) +22% > > - On hikey with performance governor > > ./perf bench sched pipe -T -l 50000 > mainline w/ patch > # migrations 0 0 > ops/sec 47721(+/-0.76%) 47899(+/- 0.56) +0.4% > > According to test on hikey, the patch doesn't impact symmetric system > compared to current implementation (only tested on arm64) > > Also read the uclamped value of task's utilization at most twice instead > instead each time we compare task's utilization with cpu's capacity. > > Fixes: b7a331615d25 ("sched/fair: Add asymmetric CPU capacity wakeup scan") > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Other than the below, I quite like this!
> --- > Changes in v2: > - merge asymmetric and symmetric path instead of duplicating tests on target, > prev and other special cases. > > - factorize call to uclamp_task_util(p) and use fits_capacity(). This could > explain part of the additionnal improvement compared to v1 (+22% instead of > +17% on v1). > > - Keep using LLC instead of asym domain for early check of target, prev and > recent_used_cpu to ensure cache sharing between the task. This doesn't > change anything for dynamiQ but will ensure same cache for legacy big.LITTLE > and also simply the changes. >
On legacy big.LITTLE systems, sd_asym_cpucapacity spans all CPUs, so we would iterate over those in select_idle_capacity() anyway - the policy we've been going for is that capacity fitness trumps cache use.
This does require the system to have a decent interconnect, cache snooping & co, but that is IMO a requirement of any sane asymmetric system.
To put words into code, this is the kind of check I would see:
if (static_branch_unlikely(&sched_asym_cpucapacity)) return fits_capacity(task_util, capacity_of(cpu)); else return cpus_share_cache(cpu, other);
> - don't check capacity for the per-cpu kthread UC because the assumption is > that the wakee queued work for the per-cpu kthread that is now complete and > the task was already on this cpu. > > - On an asymmetric system where an exclusive cpuset defines a symmetric island, > task's load is synced and tested although it's not needed. But taking care of > this special case by testing if sd_asym_cpucapacity is not null impacts by > more than 4% the performance of default sched_asym_cpucapacity path. > > - The huge increase of the number of migration for hikey960 mainline comes from > teh fact that the ftrace buffer was overloaded by events in the tests done > with v1.
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