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From: Yan Zhai <yan@cloudflare.com>
Date: Fri, 1 Mar 2024 11:30:29 -0600
Message-ID: <CAO3-PboZwTiSmVxVFFfAm94o+LgK=rnm1vbJvMhzSGep+RYzaQ@mail.gmail.com>
Subject: Re: [PATCH v2] net: raise RCU qs after each threaded NAPI poll
To: Eric Dumazet <edumazet@google.com>
Cc: 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, Joel Fernandes <joel@joelfernandes.org>, 
	"Paul E. McKenney" <paulmck@kernel.org>, =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>, 
	Alexei Starovoitov <alexei.starovoitov@gmail.com>, Steven Rostedt <rostedt@goodmis.org>, mark.rutland@arm.com
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Hi Eric,

On Fri, Mar 1, 2024 at 2:30=E2=80=AFAM Eric Dumazet <edumazet@google.com> w=
rote:
>
> I could not see the reason for 1sec (HZ) delays.
>
> Would calling rcu_softirq_qs() every ~10ms instead be a serious issue ?
>
The trouble scenarios are often when we need to detach an ad-hoc BPF
tracing program, or restart a monitoring service. It is fine as long
as they do not block for 10+ seconds or even completely stall under
heavy traffic. Raising a QS every few ms or HZ both work in such
cases.

> In anycase, if this all about rcu_tasks, I would prefer using a macro
> defined in kernel/rcu/tasks.h
> instead of having a hidden constant in a networking core function.

Paul E. McKenney was suggesting either current form or

         local_bh_enable();
         if (!IS_ENABLED(CONFIG_PREEMPT_RT))
                 rcu_softirq_qs_enable(local_bh_enable());
         else
                 local_bh_enable();

With an interval it might have to be
"rcu_softirq_qs_enable(local_bh_enable(), &next_qs);" to avoid an
unnecessary extern/static var. Will it make more sense to you?

thanks

>
> Thanks.