linux-kernel - Re: [PATCH rcu 3/3] rcu: Simplify rcu_init

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Message-ID: <4788d77e-71b5-00a2-d55a-e6a7fe60202a@huawei.com>
Date:   Thu, 1 Sep 2022 19:36:25 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
CC:     <rcu@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling



On 2022/9/1 19:11, Frederic Weisbecker wrote:
> On Thu, Sep 01, 2022 at 03:25:20AM -0700, Paul E. McKenney wrote:
>> On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
>>>> +#elif defined(CONFIG_NO_HZ_FULL)
>>>> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
>>>> +		cpumask = tick_nohz_full_mask;
>>>> +#endif
>>>
>>> A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
>>> now override nohz_full=
>>>
>>> I don't mind, it's probably what we want in the end, but the changelog should
>>> tell about it, or even better, this should be a separate change.
>>
>> Good point.  Perhaps the key point is that if there is nohz_full=,
>> rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
>> to include at least those bits set by nohz_full=.
> 
> Not sure I get what you mean. nohz_full= should in any case always force
> rcu_nocbs at least on the nohz_full CPUs.
> 
> For example assuming the following combination: rcu_nocbs=6, nohz_full=7 AND
> CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the result should be:
> 
> NOCB CPUs = 6,7
> NOHZ_FULL CPUs = 7
> 
> (CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y is overriden by rcu_nocbs=6).
> 
> Now if we have nohz_full=7 AND CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the
> result is expected to be either:
> 
> NOCB CPUs = 7 (upstream behaviour)
> NOHZ_FULL CPUs = 7

OK, I got it. CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes effect only when there
are no 'rcu_nocbs=' and 'nohz_full='.

> 
> or 
> 
> NOCB CPUs = all
> NOHZ_FULL CPUs = 7
> 
> The second makes more sense IMHO but that should be in a separate change.

OK, in this case, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes effect only when there
is no 'rcu_nocbs='.

> 
> Thanks.
> .
> 

-- 
Regards,
  Zhen Lei