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

[Beata Michalska <beata.michalska@arm.com>]

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@arm.com> 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@arm.com> 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

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

Considering only range would mean loosing the 2 (M) CPUs out of sight
for feec in some cases.

---
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@arm.com>
> > > > Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> > > > ---
> > > >  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
> > > >