linux-kernel - Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

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Date: Wed, 28 Feb 2024 17:10:43 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: Yan Zhai <yan@...udflare.com>, Eric Dumazet <edumazet@...gle.com>,
 netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Jiri Pirko <jiri@...nulli.us>, Simon Horman <horms@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Lorenzo Bianconi <lorenzo@...nel.org>, Coco Li <lixiaoyan@...gle.com>,
 Wei Wang <weiwan@...gle.com>, Alexander Duyck <alexanderduyck@...com>,
 Hannes Frederic Sowa <hannes@...essinduktion.org>,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org, bpf@...r.kernel.org,
 kernel-team@...udflare.com, rostedt@...dmis.org, mark.rutland@....com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll



> On Feb 28, 2024, at 4:52 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Wed, Feb 28, 2024 at 04:27:47PM -0500, Joel Fernandes wrote:
>> 
>> 
>>>> On Feb 28, 2024, at 4:13 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
>>> 
>>> On Wed, Feb 28, 2024 at 03:14:34PM -0500, Joel Fernandes wrote:
>>>>> On Wed, Feb 28, 2024 at 12:18 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>>>>> 
>>>>> On Wed, Feb 28, 2024 at 10:37:51AM -0600, Yan Zhai wrote:
>>>>>> On Wed, Feb 28, 2024 at 9:37 AM Joel Fernandes <joel@...lfernandes.org> wrote:
>>>>>>> Also optionally, I wonder if calling rcu_tasks_qs() directly is better
>>>>>>> (for documentation if anything) since the issue is Tasks RCU specific. Also
>>>>>>> code comment above the rcu_softirq_qs() call about cond_resched() not taking
>>>>>>> care of Tasks RCU would be great!
>>>>>>> 
>>>>>> Yes it's quite surprising to me that cond_resched does not help here,
>>>>> 
>>>>> In theory, it would be possible to make cond_resched() take care of
>>>>> Tasks RCU.  In practice, the lazy-preemption work is looking to get rid
>>>>> of cond_resched().  But if for some reason cond_resched() needs to stay
>>>>> around, doing that work might make sense.
>>>> 
>>>> In my opinion, cond_resched() doing Tasks-RCU QS does not make sense
>>>> (to me), because cond_resched() is to inform the scheduler to run
>>>> something else possibly of higher priority while the current task is
>>>> still runnable. On the other hand, what's not permitted in a Tasks RCU
>>>> reader is a voluntary sleep. So IMO even though cond_resched() is a
>>>> voluntary call, it is still not a sleep but rather a preemption point.
>>> 
>>> From the viewpoint of Task RCU's users, the point is to figure out
>>> when it is OK to free an already-removed tracing trampoline.  The
>>> current Task RCU implementation relies on the fact that tracing
>>> trampolines do not do voluntary context switches.
>> 
>> Yes.
>> 
>>> 
>>>> So a Tasks RCU reader should perfectly be able to be scheduled out in
>>>> the middle of a read-side critical section (in current code) by
>>>> calling cond_resched(). It is just like involuntary preemption in the
>>>> middle of a RCU reader, in disguise, Right?
>>> 
>>> You lost me on this one.  This for example is not permitted:
>>> 
>>>   rcu_read_lock();
>>>   cond_resched();
>>>   rcu_read_unlock();
>>> 
>>> But in a CONFIG_PREEMPT=y kernel, that RCU reader could be preempted.
>>> 
>>> So cond_resched() looks like a voluntary context switch to me.  Recall
>>> that vanilla non-preemptible RCU will treat them as quiescent states if
>>> the grace period extends long enough.
>>> 
>>> What am I missing here?
>> 
>> That we are discussing Tasks-RCU read side section? Sorry I should have been more clear. I thought sleeping was not permitted in Tasks RCU reader, but non-sleep context switches (example involuntarily getting preempted were).
> 
> Well, to your initial point, cond_resched() does eventually invoke
> preempt_schedule_common(), so you are quite correct that as far as
> Tasks RCU is concerned, cond_resched() is not a quiescent state.

 Thanks for confirming. :-)

 - Joel



> 
>                        Thanx, Paul