Re: [RFC PATCH v2 00/17] Core scheduling v2

[Subhra Mazumdar]

On 4/26/19 3:43 AM, Mel Gorman wrote:
> On Fri, Apr 26, 2019 at 10:42:22AM +0200, Ingo Molnar wrote:
> > > It should, but it's not perfect. For example, wake_affine_idle does not
> > > take sibling activity into account even though select_idle_sibling *may*
> > > take it into account. Even select_idle_sibling in its fast path may use
> > > an SMT sibling instead of searching.
> > >
> > > There are also potential side-effects with cpuidle. Some workloads
> > > migration around the socket as they are communicating because of how the
> > > search for an idle CPU works. With SMT on, there is potentially a longer
> > > opportunity for a core to reach a deep c-state and incur a bigger wakeup
> > > latency. This is a very weak theory but I've seen cases where latency
> > > sensitive workloads with only two communicating tasks are affected by
> > > CPUs reaching low c-states due to migrations.
> > >
> > > > Clearly it doesn't.
> > > It's more that it's best effort to wakeup quickly instead of being perfect
> > > by using an expensive search every time.
> > Yeah, but your numbers suggest that for *most* not heavily interacting
> > under-utilized CPU bound workloads we hurt in the 5-10% range compared to
> > no-SMT - more in some cases.
> Indeed, it was higher than expected and we can't even use the excuse that
> more resources are available to a single logical CPU as the scheduler is
> meant to keep them apart.
>
> > So we avoid a maybe 0.1% scheduler placement overhead but inflict 5-10%
> > harm on the workload, and also blow up stddev by randomly co-scheduling
> > two tasks on the same physical core? Not a good trade-off.
> >
> > I really think we should implement a relatively strict physical core
> > placement policy in the under-utilized case, and resist any attempts to
> > weaken this for special workloads that ping-pong quickly and benefit from
> > sharing the same physical core.
> It's worth a shot at least. Changes should mostly be in the wake_affine
> path for most loads of interest.
Doesn't select_idle_sibling already try to do that by calling
select_idle_core? For our OLTP workload we infact found the cost of
select_idle_core was actually hurting more than it helped to find a fully
idle core, so a net negative.