linux-kernel - Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans the old data

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Date:   Mon, 2 Oct 2023 14:44:05 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
        qyousef@...alina.io, wvw@...gle.com
Subject: Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans
 the old data



On 9/29/23 13:59, Rafael J. Wysocki wrote:
> On Fri, Sep 29, 2023 at 11:36 AM Lukasz Luba <lukasz.luba@....com> wrote:

[snip]

>> We had a few internal reviews and there were voices where saying that
>> it's better to have 2 identical tables: 'default_table' and
>> 'runtime_table' to make sure it's visible everywhere when it's used.
>> That made the need to actually have also the 'state' table inside.
>> I don't see it as a big problem, though.
> 
> What I'm trying to say is that you can allocate runtime_table along
> with the table pointed to by its state field in one invocation of
> kzalloc() (say).
> 
> Having just one memory region to free eventually instead of two of
> them would help to avoid some complexity, especially in the next
> patch.

I think, I know what you mean, basically:

------------------------------
struct em_perf_table {
	struct rcu_head rcu;
	struct em_perf_state state[];
}

kzalloc(sizeof(struct em_perf_table) + N * sizeof(struct em_perf_state))

------

IMO that should also be OK in the rest of places.
I agree the alloc/free code would be smaller.

Let me do that than.

> 
>>>
>>>> +}
>>>> +
>>>> +static void em_perf_runtime_table_set(struct device *dev,
>>>> +                                     struct em_perf_table *runtime_table)
>>>> +{
>>>> +       struct em_perf_domain *pd = dev->em_pd;
>>>> +       struct em_perf_table *tmp;
>>>> +
>>>> +       tmp = pd->runtime_table;
>>>> +
>>>> +       rcu_assign_pointer(pd->runtime_table, runtime_table);
>>>> +
>>>> +       em_cpufreq_update_efficiencies(dev, runtime_table->state);
>>>> +
>>>> +       /* Don't free default table since it's used by other frameworks. */
>>>
>>> Apparently, some frameworks are only going to use the default table
>>> while the runtime-updatable table will be used somewhere else at the
>>> same time.
>>>
>>> I'm not really sure if this is a good idea.
>>
>> Runtime table is only for driving the task placement in the EAS.
>>
>> The thermal gov IPA won't make better decisions because it already
>> has the mechanism to accumulate the error that it made.
>>
>> The same applies to DTPM, which works in a more 'configurable' way,
>> rather that hard optimization mechanism (like EAS).
> 
> My understanding of the above is that the other EM users don't really
> care that much so they can get away with using the default table all
> the time, but EAS needs more accuracy, so the table used by it needs
> to be adjusted in certain situations.

Yes

> 
> Fair enough, I'm assuming that you've done some research around it.
> Still, this is rather confusing.

Yes, I have presented those ~2y ago in Android Gerrit world
(got feedback from a few vendors) and in a few Linux conferences.

For now we don't plan to have this feature for the thermal
governor or something similar.

> 
>>>
>>>> +       if (tmp != pd->default_table)
>>>> +               call_rcu(&tmp->rcu, em_destroy_rt_table_rcu);
>>
>> The em_destroy_rt_table_rcu() is used here ^^^^^^