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From: Joel Fernandes <joel@joelfernandes.org>
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Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
Date: Wed, 28 Feb 2024 17:10:43 -0500
Message-Id: <3D27EFEF-0452-4555-8277-9159486B41BF@joelfernandes.org>
References: <02913b40-7b74-48b3-b15d-53133afd3ba6@paulmck-laptop>
Cc: Yan Zhai <yan@cloudflare.com>, Eric Dumazet <edumazet@google.com>,
 netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
 Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
 Jiri Pirko <jiri@resnulli.us>, Simon Horman <horms@kernel.org>,
 Daniel Borkmann <daniel@iogearbox.net>,
 Lorenzo Bianconi <lorenzo@kernel.org>, Coco Li <lixiaoyan@google.com>,
 Wei Wang <weiwan@google.com>, Alexander Duyck <alexanderduyck@fb.com>,
 Hannes Frederic Sowa <hannes@stressinduktion.org>,
 linux-kernel@vger.kernel.org, rcu@vger.kernel.org, bpf@vger.kernel.org,
 kernel-team@cloudflare.com, rostedt@goodmis.org, mark.rutland@arm.com
In-Reply-To: <02913b40-7b74-48b3-b15d-53133afd3ba6@paulmck-laptop>
To: paulmck@kernel.org


> On Feb 28, 2024, at 4:52=E2=80=AFPM, Paul E. McKenney <paulmck@kernel.org>=
 wrote:
>=20
> =EF=BB=BFOn Wed, Feb 28, 2024 at 04:27:47PM -0500, Joel Fernandes wrote:
>>=20
>>=20
>>>> On Feb 28, 2024, at 4:13=E2=80=AFPM, Paul E. McKenney <paulmck@kernel.o=
rg> wrote:
>>>=20
>>> =EF=BB=BFOn Wed, Feb 28, 2024 at 03:14:34PM -0500, Joel Fernandes wrote:=

>>>>> On Wed, Feb 28, 2024 at 12:18=E2=80=AFPM Paul E. McKenney <paulmck@ker=
nel.org> wrote:
>>>>>=20
>>>>> On Wed, Feb 28, 2024 at 10:37:51AM -0600, Yan Zhai wrote:
>>>>>> On Wed, Feb 28, 2024 at 9:37=E2=80=AFAM Joel Fernandes <joel@joelfern=
andes.org> wrote:
>>>>>>> Also optionally, I wonder if calling rcu_tasks_qs() directly is bett=
er
>>>>>>> (for documentation if anything) since the issue is Tasks RCU specifi=
c. Also
>>>>>>> code comment above the rcu_softirq_qs() call about cond_resched() no=
t taking
>>>>>>> care of Tasks RCU would be great!
>>>>>>>=20
>>>>>> Yes it's quite surprising to me that cond_resched does not help here,=

>>>>>=20
>>>>> 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 ri=
d
>>>>> of cond_resched().  But if for some reason cond_resched() needs to sta=
y
>>>>> around, doing that work might make sense.
>>>>=20
>>>> 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.
>>>=20
>>> =46rom 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.
>>=20
>> Yes.
>>=20
>>>=20
>>>> 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?
>>>=20
>>> You lost me on this one.  This for example is not permitted:
>>>=20
>>>   rcu_read_lock();
>>>   cond_resched();
>>>   rcu_read_unlock();
>>>=20
>>> But in a CONFIG_PREEMPT=3Dy kernel, that RCU reader could be preempted.
>>>=20
>>> 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.
>>>=20
>>> What am I missing here?
>>=20
>> That we are discussing Tasks-RCU read side section? Sorry I should have b=
een more clear. I thought sleeping was not permitted in Tasks RCU reader, bu=
t non-sleep context switches (example involuntarily getting preempted were).=

>=20
> 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


>=20
>                        Thanx, Paul