Messages in this thread | | | Date | Fri, 15 Jul 2016 17:02:53 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain |
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On Fri, Jul 15, 2016 at 03:39:05PM +0200, Vincent Guittot wrote: > On 15 July 2016 at 13:46, Morten Rasmussen <morten.rasmussen@arm.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@arm.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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