Re: [PATCH v4 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched_domain flag

From: Vincent Guittot
Date: Tue May 18 2021 - 11:56:37 EST

On Tue, 18 May 2021 at 17:48, Beata Michalska <beata.michalska@xxxxxxx> wrote:
>
> On Tue, May 18, 2021 at 05:28:11PM +0200, Vincent Guittot wrote:
> > On Tue, 18 May 2021 at 17:09, Beata Michalska <beata.michalska@xxxxxxx> wrote:
> > >
> > > On Tue, May 18, 2021 at 04:53:09PM +0200, Vincent Guittot wrote:
> > > > On Tue, 18 May 2021 at 16:27, Beata Michalska <beata.michalska@xxxxxxx> wrote:
> > > > >
> > > > > On Tue, May 18, 2021 at 03:39:27PM +0200, Vincent Guittot wrote:
> > > > > > On Mon, 17 May 2021 at 10:24, Beata Michalska <beata.michalska@xxxxxxx> wrote:
> > > > > > >
> > > > > > > Introducing new, complementary to SD_ASYM_CPUCAPACITY, sched_domain
> > > > > > > topology flag, to distinguish between shed_domains where any CPU
> > > > > > > capacity asymmetry is detected (SD_ASYM_CPUCAPACITY) and ones where
> > > > > > > a full range of CPU capacities is visible to all domain members
> > > > > > > (SD_ASYM_CPUCAPACITY_FULL).
> > > > > >
> > > > > > I'm not sure about what you want to detect:
> > > > > >
> > > > > > Is it a sched_domain level with a full range of cpu capacity, i.e.
> > > > > > with at least 1 min capacity and 1 max capacity ?
> > > > > > or do you want to get at least 1 cpu of each capacity ?
> > > > > That would be at least one CPU of each available capacity within given domain,
> > > > > so full -set- of available capacities within a domain.
> > > >
> > > > Would be good to add the precision.
> > > Will do.
> > > >
> > > > Although I'm not sure if that's the best policy compared to only
> > > > getting the range which would be far simpler to implement.
> > > > Do you have some topology example ?
> > >
> > > An example from second patch from the series:
> > >
> > > DIE [ ]
> > > MC [ ][ ]
> > >
> > > CPU [0] [1] [2] [3] [4] [5] [6] [7]
> > > Capacity |.....| |.....| |.....| |.....|
> > > L M B B
> >
> > The one above , which is described in your patchset, works with the range policy
> Yeap, but that is just a variation of all the possibilities....
> >
> > >
> > > Where:
> > > arch_scale_cpu_capacity(L) = 512
> > > arch_scale_cpu_capacity(M) = 871
> > > arch_scale_cpu_capacity(B) = 1024
> > >
> > > which could also look like:
> > >
> > > DIE [ ]
> > > MC [ ][ ]
> > >
> > > CPU [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
> > > Capacity |.....| |.....| |.....| |.....| |.....|
> > > L M B L B
> >
> > I know that that HW guys can come with crazy idea but they would
> > probably add M instead of L with B in the 2nd cluster as a boost of
> > performance at the cost of powering up another "cluster" in which case
> > the range policy works as well
> >
> > >
> > > Considering only range would mean loosing the 2 (M) CPUs out of sight
> > > for feec in some cases.
> >
> > Is it realistic ? Considering all the code and complexity added by
> > patch 2, will we really use it at the end ?
> >
> I do completely agree that the first approach was slightly .... blown out of
> proportions, but with Peter's idea, the complexity has dropped significantly.
> With the range being considered we are back to per domain tracking of available
> capacities (min/max), plus additional cycles on comparing capacities.
> Unless I fail to see the simplicity of that approach ?

With the range, you just have to keep track of one cpumask for min
capacity and 1 for max capacity (considering that the absolute max
capacity/1024 might not be in the cpumap) instead of tracking all
capacity and manipulating/updating a dynamic link list. Then as soon
as you have 1 cpu of both masks then you are done. As a 1st glance
this seems to be simpler to do.

>
> ---
> BR
> B.
> > Regards,
> > Vincent
> > >
> > > ---
> > > BR.
> > > B
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > >
> > > > > ---
> > > > > BR
> > > > > B.
> > > > > >
> > > > > >
> > > > > > >
> > > > > > > With the distinction between full and partial CPU capacity asymmetry,
> > > > > > > brought in by the newly introduced flag, the scope of the original
> > > > > > > SD_ASYM_CPUCAPACITY flag gets shifted, still maintaining the existing
> > > > > > > behaviour when one is detected on a given sched domain, allowing
> > > > > > > misfit migrations within sched domains that do not observe full range
> > > > > > > of CPU capacities but still do have members with different capacity
> > > > > > > values. It loses though it's meaning when it comes to the lowest CPU
> > > > > > > asymmetry sched_domain level per-cpu pointer, which is to be now
> > > > > > > denoted by SD_ASYM_CPUCAPACITY_FULL flag.
> > > > > > >
> > > > > > > Signed-off-by: Beata Michalska <beata.michalska@xxxxxxx>
> > > > > > > Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> > > > > > > ---
> > > > > > > include/linux/sched/sd_flags.h | 10 ++++++++++
> > > > > > > 1 file changed, 10 insertions(+)
> > > > > > >
> > > > > > > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> > > > > > > index 34b21e9..57bde66 100644
> > > > > > > --- a/include/linux/sched/sd_flags.h
> > > > > > > +++ b/include/linux/sched/sd_flags.h
> > > > > > > @@ -91,6 +91,16 @@ SD_FLAG(SD_WAKE_AFFINE, SDF_SHARED_CHILD)
> > > > > > > SD_FLAG(SD_ASYM_CPUCAPACITY, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > > > > > >
> > > > > > > /*
> > > > > > > + * Domain members have different CPU capacities spanning all unique CPU
> > > > > > > + * capacity values.
> > > > > > > + *
> > > > > > > + * SHARED_PARENT: Set from the topmost domain down to the first domain where
> > > > > > > + * all available CPU capacities are visible
> > > > > > > + * NEEDS_GROUPS: Per-CPU capacity is asymmetric between groups.
> > > > > > > + */
> > > > > > > +SD_FLAG(SD_ASYM_CPUCAPACITY_FULL, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > > > > > > +
> > > > > > > +/*
> > > > > > > * Domain members share CPU capacity (i.e. SMT)
> > > > > > > *
> > > > > > > * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share
> > > > > > > --
> > > > > > > 2.7.4
> > > > > > >