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From: Yan Zhai <yan@cloudflare.com>
Date: Wed, 28 Feb 2024 10:37:51 -0600
Message-ID: <CAO3-Pboo32iQBBUHUELUkvvpSa=jZwUqefrwC-NBjDYx4yxYJQ@mail.gmail.com>
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll
To: Joel Fernandes <joel@joelfernandes.org>
Cc: paulmck@kernel.org, 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
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On Wed, Feb 28, 2024 at 9:37=E2=80=AFAM Joel Fernandes <joel@joelfernandes.=
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. Al=
so
> code comment above the rcu_softirq_qs() call about cond_resched() not tak=
ing
> care of Tasks RCU would be great!
>
Yes it's quite surprising to me that cond_resched does not help here,
I will include that comment. Raising just the task RCU QS seems
sufficient to the problem we encountered. But according to commit
d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt
when safe"), there might be additional threat factor in __do_softirq
that also applies to threaded poll.

Yan


> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> thanks,
>
>  - Joel
> [1]
> @@ -381,8 +553,10 @@ asmlinkage __visible void __softirq_entry __do_softi=
rq(void)
>                 pending >>=3D softirq_bit;
>         }
>
> -       if (__this_cpu_read(ksoftirqd) =3D=3D current)
> +       if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> +           __this_cpu_read(ksoftirqd) =3D=3D current)
>                 rcu_softirq_qs();
> +
>         local_irq_disable();