linux-kernel - Re: [PATCH 2/2] context_tracking, rcu: Rename RCU_DYNTICKS_IDX to CT_DYNTICKS

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Date: Tue, 16 Apr 2024 10:36:18 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: paulmck@...nel.org, rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
 Peter Zijlstra <peterz@...radead.org>, Neeraj Upadhyay
 <quic_neeraju@...cinc.com>, Joel Fernandes <joel@...lfernandes.org>, Josh
 Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>,
 Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH 2/2] context_tracking, rcu: Rename RCU_DYNTICKS_IDX to
 CT_DYNTICKS_IDX

On 15/04/24 23:08, Frederic Weisbecker wrote:
> Le Mon, Apr 15, 2024 at 06:36:31PM +0200, Valentin Schneider a écrit :
>>
>> Sounds good to me too, thanks for the suggestion :)
>>
>> Now, what about ct_dynticks() & friends? I was about to do:
>>
>> -static __always_inline int ct_dynticks(void)
>> +static __always_inline int ct_rcu_watching(void)
>>  {
>> -	return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_DYNTICKS_MASK;
>> +	return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_RCU_WATCHING_MASK;
>>  }
>
> Yup!
>
>>
>> ... but then realised that there's more siblings to the rcu_dynticks*()
>> family;
>
> Ouch right, sorry I forgot there is so much of this namespace. But in case you're
> willing to clean that up:
>

While I'm at it, I figure I might as well.

>>
>> AFAICT dynticks_nesting could also get the rcu_watching prefix treatment,
>> `rcu_dynticks_task_exit() -> rcu_watching_task_exit` doesn't sound as
>
> rcu_tasks_exit() ?
>
> But Paul, is there a reason why check_holdout_task() doesn't check
> ct_dynticks_cpu(task_cpu(t)) instead of maintaining this separate counter?
>
>> obvious though. The rcu_dyntick event probably can't be renamed either.
>
> I think we can rename trace_rcu_dyntick() to trace_rcu_watching()
>
>>
>> I'm not sure how far to take the renaming; seeing things like:
>>
>>   notrace bool rcu_is_watching(void)
>>   {
>>           bool ret;
>>
>>           preempt_disable_notrace();
>>           ret = !rcu_dynticks_curr_cpu_in_eqs();
>>           preempt_enable_notrace();
>>           return ret;
>>   }
>>   EXPORT_SYMBOL_GPL(rcu_is_watching);
>>
>> makes me think most of the rcu_*dynticks / rcu_*eqs stuff could get an
>> rcu_watching facelift?
>
> The eqs part can stay as-is. But the *dynticks* needs an update.
>
>>
>> Here are my current considerations for identifiers used in context_tracking
>> in decreasing order of confidence:
>>
>> | Old                                   | New                                                           |
>> |---------------------------------------+---------------------------------------------------------------|
>> | RCU_DYNTICKS_IDX                      | CT_RCU_WATCHING                                               |
>> | RCU_DYNTICKS_MASK                     | CT_RCU_WATCHING_MASK                                          |
>> | context_tracking.dynticks_nesting     | context_tracking.rcu_watching_nesting                         |
>
> This can be context_tracking.nesting (and yes one day we might need to lock up
> context_tracking.nesting and context_tracking.recursion together in a room and see
> who wins after a day or two).
>

Much better!

>> | context_tracking.dynticks_nmi_nesting | context_tracking.rcu_watching_nmi_nesting [bit of a mouthful] |
>
> context_tracking.nmi_nesting
>
>> | rcu_dynticks_curr_cpu_in_eqs()        | rcu_watching_curr_cpu() [with an added negation]              |
>
> Nice!
>
>> |---------------------------------------+---------------------------------------------------------------|
>> | TRACE_EVENT_RCU(rcu_dyntick,          | [Can't change?]                                               |
>
> It can change. Officially trace events aren't ABI. Unoficially I wouldn't dare
> changing the sched switch trace event but this one is fine.
>

Cool, away it goes then :)

>> |---------------------------------------+---------------------------------------------------------------|
>> | rcu_dynticks_task_enter()             | rcu_watching_task_enter()> | |
>
> rcu_tasks_enter() ?
>
>> | rcu_dynticks_task_exit()              | rcu_watching_task_exit()                                      |
>
> rcu_tasks_exit() ?
>
>> | rcu_dynticks_task_trace_enter()       | rcu_watching_task_trace_enter()                               |
>
> rcu_tasks_trace_enter()?
>
>> | rcu_dynticks_task_trace_exit()        | rcu_watching_task_trace_exit()                                |
>
> rcu_tasks_trace_exit() ?
>

Now that you point it out, it looks obvious!

> Thanks.
>
>>
>> Thoughts?
>>