Re: [PATCHv3 1/9] sched: Add static_key for asymmetric cpu capacity optimizations

[Morten Rasmussen]

On Thu, Jun 28, 2018 at 07:16:46PM +0200, Dietmar Eggemann wrote:
> On 06/28/2018 10:48 AM, Morten Rasmussen wrote:
> >On Wed, Jun 27, 2018 at 05:41:22PM +0200, Dietmar Eggemann wrote:
> >>On 06/22/2018 04:36 PM, Morten Rasmussen wrote:
> >>>On Fri, Jun 22, 2018 at 09:22:22AM +0100, Quentin Perret wrote:
> 
> [...]
> 
> >>>>What would happen if you hotplugged an entire cluster ? You'd loose the
> >>>>SD_ASYM_CPUCAPACITY flag but keep the static key is that right ? Should
> >>>>we care ?
> >>>
> >>>I don't think we should care. The static key enables additional checks
> >>>and tweaks but AFAICT none of them requires the SD_ASYM_CPUCAPACITY to
> >>>be set and they should all be have no effect if that is the case. I
> >>>added the static key just avoid the overhead on systems where they would
> >>>have no effect. At least that is intention, I could course have broken
> >>>things by mistake.
> >>
> >>I tent to agree for misfit but it would be easy to just add an
> >>static_branch_disable_cpuslocked() into the else path of if(enable).
> >
> >Depending on how we address the exclusive cpuset mess Quentin pointed
> >out, it isn't as simple as that. As it is with our current
> >not-yet-posted patches we would never remove the SD_ASYM_CPUCAPACITY
> >flag, so we would never do a static_branch_disable_cpuslocked().
> 
> I was referring rather to the 'hotplug out an entire cluster' mentioned
> above. Looking closer into the code, I see that this will only work for
> traditional big.LITTLE (one big, one little cluster) since on them the DIE
> level vanishes and so the SD_ASYM_CPUCAPACITY flag.

Agreed, disabling the branch in that case should be possible but only if
we know that there aren't any exclusive cpusets.

> Currently we only detect the flags when the system comes up (in the
> init_cpu_capacity_callback()) and not when we hotplug cpus. That's why it
> doesn't work for your 'three cluster system where 0-3 and 4-7 little
> clusters, and 8-11 is a big cluster' example.
> So we don't re-detect the flags every time we call from the scheduler into
> the arch code and so the the DIE level arch topology flag function will
> return SD_ASYM_CPUCAPACITY.

It would fairly easy to re-detect every time we hotplug a cpu in/out but
that won't help us with exclusive cpuset issue.

> 
> >However, I'm currently thinking that we should probably set the flag
> >according to the cpus in each root_domain. If we do that, we can't just
> >disable the static_key because one root_domain is SMP, so we would have
> >to track if _any_ root_domain (exclusive cpuset topology) has the flag
> >set.
> 
> This is then in sync with the energy model where the static key should be
> enabled if any root domain can do EAS. The static key would be system wide,
> not per root domain.

The static_key can only be system wide :-)

> 
> >Is it worth it to iterate over all the exclusive cpuset with all
> >the locking implications to disable the static_key in the corner case
> >where exclusive cpuset are set up for all cpu capacities in the system?
> 
> Don't know either? But if the code to do this would include 'exclusive
> cpusets' and platforms like your three cluster example then maybe ...

Iterating over all root_domains should work for any platform. IMHO, it
might be a lot of additional complexity to disable some optimizations in
a few corner cases.

> 
> [...]
> 
> >>>I'm tempted to say we shouldn't care in this situation. Setting the
> >>>flags correctly in the three cluster example would require knowledge
> >>>about the cpuset configuration which we don't have in the arch code so
> >>>SD_ASYM_CPUCAPACITY flag detection would have be done by the
> >>>sched_domain build code. However, not setting the flag according to the
> >>>actual members of the exclusive cpuset means that homogeneous
> >>>sched_domains might have SD_ASYM_CPUCAPACITY set enabling potentially
> >>>wrong scheduling decisions.
> >>
> >>We could easily pass the CPU as an argument to all these
> >>sched_domain_flags_f functions.
> >>
> >>-typedef int (*sched_domain_flags_f)(void);
> >>+typedef int (*sched_domain_flags_f)(int cpu);
> >>
> >>In this case, the arch specific flag functions on a sched domain (sd) level
> >>could use the corresponding sched_domain_mask_f function to iterate over the
> >>span of the sd seen by CPU instead of all online cpus.
> >
> >Yeah, but I'm afraid that doesn't solve the problem. The
> >sched_domain_mask_f function doesn't tell us anything about any
> >exclusive cpusets. We need sched_domain_span(sd) which is
> 
> You're right, I checked again and even if we hotplug, the flag function
> tl->mask(cpu) like cpu_coregroup_mask() always return the initial set of
> cpus. So we are only save if the DIE sd vanishes and with it the
> SD_ASYM_CPUCAPACITY flag since nobody will call the appropriate arch
> topology flag function.

Yeah, that isn't really solving the problem, it is more like working by
accident ;-)

> 
> >sched_domain_mask_f & cpu_map where cpu_map is the cpumask of the
> >exclusive cpuset. So we either need to pass the sched_domain_span() mask
> >through sched_domain_flags_f or work our way back to that information
> >based on the cpu id. I'm not sure if the latter is possible.
> 
> Agreed.
> 
> [...]
> 
> >>We could also say that systems with 2 clusters with the same uArch and same
> >>max CPU frequency and additional clusters are insane, like we e.g. do with
> >>the Energy Model and CPUs with different uArch within a frequency domain?
> >
> >I fail to get the point here. Are you saying that SMP is insane? ;-)
> 
> The '... and additional clusters' is important, a system like your three
> cluster system you describe above.
> 
> But the problem that if you hotplug-out the big cluster, the DIE level is
> still there with the SD_ASYM_CPUCAPACITY flag is due to the fact that we
> don't start the flag detection mechanism during hotplug, right?

True, flag detection is currently only done once (well, twice: when the
secondary cpus come up, and again at cpufreq init). We can't rely on
sched_domains being elemitated to get the flags right in the general
case. It just appears to work on systems we have considered so far.

> 
> >>>We can actually end up with this problem just by hotplugging too. If you
> >>>unplug the entire big cluster in the three cluster example above, you
> >>>preserve DIE level which would have SD_ASYM_CPUCAPACITY set even though
> >>>we only have little cpus left.
> 
> IMHO, again that's because we do flag detection only at system startup.

Yes.

> 
> >>>As I see it, we have two choices: 1) Set the flags correctly for
> >>>exclusive cpusets which means some additional "fun" in the sched_domain
> >>>hierarchy set up, or 2) ignore it and make sure that setting
> >>
> >>I assume you refer to this cpu parameter for sched_domain_flags_f under 1).
> >
> >No, what I had in mind was to let sd_init() set SD_ASYM_CPUCAPACITY
> >flag as the arch-code doesn't know about the exclusive cpusets as
> >discussed above. In the meantime I have thought about a different
> >approach where sd_init() only disables the flag when it is no longer
> >needed due to exclusive cpusets.
> 
> In this case sd_init() has to know the cpu capacity values. I assume that
> the arch still has to call rebuild_sched_domains() after cpufreq is up and
> running due to the max cpu frequency dependency for cpu capacity.

Yes, sd_init() will just access rq->cpu_capacity_orig which should be
straightforward. There is no change in when we rebuild the sched_domain
hierarchy. It will be rebuild once cpufreq is up, and every time we mess
with exclusive cpusets.