Re: [PATCH] PM / EM: consult something about cpumask in em_dev_register_perf_domain

[zhuguangqing83]

> 
> Hi,
> 
> On Monday 12 Oct 2020 at 20:41:36 (+0800), zhuguangqing83@xxxxxxxxx wrote:
> > From: zhuguangqing <zhuguangqing@xxxxxxxxxx>
> >
> > Hi, Lukasz, Quentin
> >   I have three questions to consult about cpumask in energy_model.c.
> 
> OK, let's see if we can help :)
> 

Thanks for your help and review.

> > 1, The first one is about the meanings of the following two parameters,
> > [1] and [2].
> > [1]: "cpumask_t *cpus" in function em_dev_register_perf_domain():
Pointer
> > to cpumask_t, which in case of a CPU device is obligatory. It can be
taken
> > from i.e. 'policy->cpus'.
> > [2]: "unsigned long cpus[]" in struct em_perf_domain: Cpumask covering
the
> > CPUs of the domain. It's here for performance reasons to avoid potential
> > cache misses during energy calculations in the scheduler and simplifies
> > allocating/freeing that memory region.
> >
> > From the current code, we see [2] is copied from [1]. But from comments,
> > accorinding to my understanding, [2] and [1] have different meanings.
> > [1] can be taken from i.e. 'policy->cpus', according to the comment in
the
> > defination of struct cpufreq_policy, it means Online CPUs only.
Actually,
> > 'policy->cpus' is not always Online CPUs.
> > [2] means each_possible_cpus in the same domain, including phycical
> > hotplug cpus(just possible), logically hotplug cpus(just present) and
> > online cpus.
> >
> >
> > So, the first question is, what are the meanings of [1] and [2]?
> > I guess maybe there are the following 4 possible choices.
> > A), for_each_possible_cpu in the same domain, maybe phycical hotplug
> > B), for_each_present_cpu in the same domain, maybe logically hotplug
> > C), for_each_online_cpu in the same domain, online
> > D), others
> 
> So, if the comments are confusing we should update them, but from the EM
> framework perspective, all cpumasks must be the _possible_ CPUs in the
> domain. It's up to the clients (e.g. the scheduler) to deal with hotplug
> and so on, but the EM framework should cover non-online CPUs too.
> 

I see, from the EM framework perspective, all cpumasks must be the
_possible_ CPUs in the domain. But up to the clients (e.g. the scheduler),
'policy->cpus' maybe not the _possible_ CPUs. For example, in the function
scmi_cpufreq_init() which calls em_dev_register_perf_domain(),
'policy->cpus'
is got from scmi_get_sharing_cpus() and cpufreq_online(), including CPUs
satisfied the following three conditions at the same time which means
_present_ CPUs according to my understanding.
1) _possible_
2) if (get_cpu_device(cpu))
3) in the same domain

> > 2, The second question is about the function
em_dev_register_perf_domain().
> > If multiple clients register the same performance domain with different
> > *dev or *cpus, how should we handle?
> >
> > int em_dev_register_perf_domain(struct device *dev, unsigned int
nr_states,
> > 			struct em_data_callback *cb, cpumask_t *cpus)
> >
> > For example, say cpu0 and cpu1 are in the same performance domain,
> > cpu0 is registered first. As part of the init process,
> > em_dev_register_perf_domain() is called, then *dev = cpu0_dev,
> > *cpus = 01b(cpu1 is initially offline). It creates a em_pd for cpu0_dev.
> > After a while, cpu1 is online, em_dev_register_perf_domain() is called
> > again as part of init process for cpu1, then *dev =cpu1_dev,
> > *cpus = 11b(cpu1 is online).  In this case, for the current code,
> > cpu1_dev can not get its em_pd.
> 
> As per the above, the registration should be done once, with the mask of
> all possible CPUs in the domain. If CPUs 0 and 1 share the same domain, a
> single call to em_dev_register_perf_domain() should be sufficient to
> register both of them at once.
> 

I just saw your discussion in https://lkml.org/lkml/2020/2/7/479, it
mentioned
"PM_EM ignores hotplug ATM. Perhaps we should document that somewhere ..."

So, if PM_EM still ignores hotplug now, then the registration should be done
once,
with the mask of all possible CPUs in the domain. If PM_EM consider hotplug
in
the future, then we should consider the case that
em_dev_register_perf_domain()
will be called more than once with different input parameters *dev or/and
*cpus.
And the CPU mask might not be all possible CPUs in the domain.

> > 3, The third question is, how can we ensure cpu_dev as follows is not
> > NULL? If we can't ensure that, maybe we should add a check before using
> > it.
> > /kernel/power/energy_model.c
> > 174) static int em_create_pd(struct device *dev, int nr_states,
> > 175)                    struct em_data_callback *cb, cpumask_t *cpus)
> > 176) {
> > 199)    if (_is_cpu_device(dev))
> > 200)            for_each_cpu(cpu, cpus) {
> > 201)                    cpu_dev = get_cpu_device(cpu);
> > 202)                    cpu_dev->em_pd = pd;
> > 203)            }
> 
> And that should not be necessary as we check for the !dev case at the
> top of em_dev_register_perf_domain(). Or were you thinking about
> something else?
> 
> Oh I think I read that one wrong, but the conclusion should be the same,
> at least on Arm64 -- all _possible_ CPUs should be registered early
> enough for that not to be an issue.
>
>Did you observe anything wrong there for your use-case?
> 

I did not observe anything wrong for my use-case. But I think it's possible
in
theory that cpu_dev maybe NULL. I observe that in the function
scmi_cpufreq_init(), before calling em_dev_register_perf_domain(),
'policy->cpus' can be ensure that all the cpu_dev in CPU mask are not NULL.
But maybe we can not ensure all the clients do the check.  This could happen
if the arch did not set up cpu_dev since this CPU is not in cpu_present mask
and the driver did not send a correct CPU mask during registration.

> Thanks,
> Quentin