Received-SPF: pass (google.com: domain of linux-kernel+bounces-85720-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From: Joel Fernandes <joel@joelfernandes.org>
Precedence: bulk
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
Date: Wed, 28 Feb 2024 16:27:47 -0500
Message-Id: <4965F5CD-B33C-4B75-818A-021372020881@joelfernandes.org>
References: <5b74968d-fe14-48b4-bb16-6cf098a04ca5@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: <5b74968d-fe14-48b4-bb16-6cf098a04ca5@paulmck-laptop>
To: paulmck@kernel.org


> On Feb 28, 2024, at 4:13=E2=80=AFPM, Paul E. McKenney <paulmck@kernel.org>=
 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@kerne=
l.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@joelfernan=
des.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 t=
aking
>>>>> 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 rid
>>> of cond_resched().  But if for some reason cond_resched() needs to stay
>>> 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.

Yes.

>=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?

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 n=
on-sleep context switches (example involuntarily getting preempted were).

 - Joel


>=20
>                            Thanx, Paul