Message-ID: <20160715160253.GH21816@e105550-lin.cambridge.arm.com>
Date:	Fri, 15 Jul 2016 17:02:53 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Yuyang Du <yuyang.du@...el.com>, mgalbraith@...e.de,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root
 domain

On Fri, Jul 15, 2016 at 03:39:05PM +0200, Vincent Guittot wrote:
> On 15 July 2016 at 13:46, Morten Rasmussen <morten.rasmussen@....com> wrote:
> > On Thu, Jul 14, 2016 at 04:15:20PM +0100, Morten Rasmussen wrote:
> >> On Thu, Jul 14, 2016 at 03:25:36PM +0200, Vincent Guittot wrote:
> >> > On 13 July 2016 at 18:37, Morten Rasmussen <morten.rasmussen@....com> wrote:
> >> > > Also, for SMT max capacity is less than 1024 already. No?
> >> >
> >> > Yes, it is. I haven't looked in details but i think that we could use
> >> > a capacity of 1024 for SMT with changes that have been done on how to
> >> > evaluate if a sched_group is overloaded or not.
> >>
> >> Changing SMT is a bit more invasive that I had hoped for for this patch
> >> set. I will see if we can make it work with the current SMT capacities.
> >>
> >> >
> >> > > But we may be able to cater for this in wake_cap() somehow. I can have a
> >> > > look if Vincent doesn't like this patch.
> >> >
> >> > IMO, rd->max_cpu_capacity field doesn't seem to be required for now .
> >>
> >> No problem. I will try to get rid of it. I will drop the "arm:" patches
> >> as well as they would have to be extended to guarantee a max capacity of
> >> 1024 and we most likely will have to change it again when Juri's DT
> >> solution hopefully gets merged.
> >
> > I have had a closer look at wake_cap() again. Getting rid of
> > rd->max_cpu_capacity isn't as easy as I thought.
> >
> > The fundamental problem is that all we have in wake_cap() is the waking
> > cpu and previous cpu ids which isn't sufficient to determine whether we
> > have an asymmetric capacity system or not. A capacity <1024 can either a
> > little cpu or an SMT thread. We need a third piece of information, which
> > can be either the highest cpu capacity available in the cpu, or a
> > flag/variable/function telling us whether we are on an SMT system.
> >
> > I see the following solutions to the problem:
> >
> > 1. Have a system-wide max_cpu_capacity (as proposed in this patch) which
> > can let us detect SMT systems as max_cpu_capacity < 1024 implies SMT.
> >
> > 2. Change SMT thread capacity to 1024 so we implicitly know that max
> > capacity is always 1024. As said above, this is a very invasive change
> > as it would mean that we no longer distinguish between SMP and SMT.
> > smt_gain and SD_SHARE_CPUCAPACITY would no longer have any effect and
> > can be ripped out. I would prefer not create a dependency on such a
> > massive change. We can do the experiment afterwards if needed.
> >
> > 3. Detect SMT in wake_cap(). This requires access to the sched_domain
> > hierarchy as the SD_SHARE_CPUCAPACITY is the only way to detect SMT,
> > AFAIK, apart from looping through the capacities of all cpus in the
> > system basically computing max_cpu_capacity each time.
> > wake_cap() is currently called before rcu_read_lock() that gives us
> > access to the sched_domain hierarchy. I would have to postpone the
> > wake_cap() call to being inside the lock and introduce another lookup in
> > the sched_domain hierarchy which would be executed on every wake-up on
> > all systems. IMHO, that is a bit ugly.
> >
> > I don't really like any of the solutions, but of those three I would go
> > for the current solution (1) as it is very minimal both in the amount of
> > code touched/affected and overhead. We can kill it later if we have a
> > better one, no problem for me.
> 
> I had solution 2 in mind. I haven't looked deeply the impact but I
> thought that the main remaining blocking  point is in
> update_numa_stats where it use the fact that the capacity is less than
> 1024 vat SMT level to compute task_capacity and  set has_free_capacity
> only if we have less than 1 task per core.
> smt_gain would not be used anymore

Isn't group capacities of also smaller and hence influence load
balancing decisions?

I was hoping that we could decouple a full audit of the load-balance
code from this relatively simple patch set by staying with 1 for now. I
worry that the changing SMT capacity can turn into a major task. Just
proving that there is no regressions even if we know it should be, is a
lot of work.

I'm happy to look at the SMT stuff it has been on my list of outstanding
issues for a very long time, but I would prefer to break it into
multiple independent patch sets to keep them focused. I haven't had a
much luck with massive complicated patch sets so far ;-)

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