Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain

[Vincent Guittot]

On 15 July 2016 at 18:02, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> On Fri, Jul 15, 2016 at 03:39:05PM +0200, Vincent Guittot wrote:
>> On 15 July 2016 at 13:46, Morten Rasmussen <morten.rasmussen@xxxxxxx> 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@xxxxxxx> 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?

It should not because the capacity is now only used to compare groups
together and no more with the 1024 value

>
> 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.

Yes, you are probably right on that point

>
> 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 ;-)