linux-kernel - Re: [PATCH] sched/numa: Restore sched feature NUMA to its earlier avatar.

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Date:	Thu, 9 Jul 2015 10:01:51 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] sched/numa: Restore sched feature NUMA to its earlier
 avatar.


* Srikar Dronamraju <srikar@...ux.vnet.ibm.com> wrote:

> > So I find the patch, the description and the comments in the code conflicting and 
> > confusing.
> > 
> > The patch does this:
> > 
> > @@ -5676,10 +5676,10 @@ static int migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
> >         unsigned long src_faults, dst_faults;
> >         int src_nid, dst_nid;
> > 
> > -       if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
> > +       if (!sched_feat(NUMA) || !sched_feat(NUMA_FAVOUR_HIGHER))
> >                 return -1;
> > 
> > -       if (!sched_feat(NUMA))
> > +       if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
> >                 return -1;
> > 
> >         src_nid = cpu_to_node(env->src_cpu);
> > 
> > 
> > while the default for 'NUMA' is 0, 'NUMA_FAVOUR_HIGHER' is 1.
> > 
> > Which in itself is confusing: WTH do we have a generic switch called 'NUMA' and 
> > then have it disabled?
> 
> NUMA feature gets enabled on multi-node boxes because of
> 
> start_kernel() -> numa_policy_init() -> check_numabalancing_enable() ->
>  set_numabalancing_state() -> sched_feat_set("NUMA");

Ugh, that is nonsensical!

If CONFIG_SCHED_DEBUG is disabled then sched_features is a constant value:

  # define const_debug const

  ...

  extern const_debug unsigned int sysctl_sched_features;

sched_features are _only_ meant for debugging. They turn into an unchangeable set 
of features when SCHED_DEBUG is disabled - and that is very much by design.

The whole set_numabalancing_state() muck needs to be fixed.

Thanks,

	Ingo
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