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Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag

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  Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag Peter Zijlstra <peterz@infradead.org> - 2016-06-01 22:20 +0200
    Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag Morten Rasmussen <morten.rasmussen@arm.com> - 2016-06-08 10:50 +0200

#1411521 — Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag

On Mon, May 23, 2016 at 11:58:49AM +0100, Morten Rasmussen wrote:
> For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
> sched_domain hierarchy we need a way to enable wake-up balancing for the
> lower levels as well as we may want to balance tasks that don't fit the
> capacity of the previous cpu.
> 
> We have the option of introducing a new topology flag to express this
> requirement, or let the existing SD_BALANCE_WAKE flag be set by the
> architecture as a topology flag. The former means introducing yet
> another flag, the latter breaks the current meaning of topology flags.
> None of the options are really desirable.

So why can't you couple this to ASYM_CAPACITY? If that's set anywhere,
add BALANCE_WAKE as appropriate?

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#1417091

On Wed, Jun 01, 2016 at 10:18:17PM +0200, Peter Zijlstra wrote:
> On Mon, May 23, 2016 at 11:58:49AM +0100, Morten Rasmussen wrote:
> > For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
> > sched_domain hierarchy we need a way to enable wake-up balancing for the
> > lower levels as well as we may want to balance tasks that don't fit the
> > capacity of the previous cpu.
> > 
> > We have the option of introducing a new topology flag to express this
> > requirement, or let the existing SD_BALANCE_WAKE flag be set by the
> > architecture as a topology flag. The former means introducing yet
> > another flag, the latter breaks the current meaning of topology flags.
> > None of the options are really desirable.
> 
> So why can't you couple this to ASYM_CAPACITY? If that's set anywhere,
> add BALANCE_WAKE as appropriate?

That should be possible. It is set at the sched_domain levels that span
cpus of different capacity, but we need to set BALANCE_WAKE on the level
below as well. So we would have to introduce a dependency between flags
set at different levels.

However, following the discussion with Vincent on enabling WAKE_AFFINE
across cpus of different capacity, I might be able to repurpose
ASYM_CPUCAPACITY to set BALANCE_WAKE instead which would be simpler I
think.

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