linux-kernel - Re: [PATCH v2] net: raise RCU qs after each threaded NAPI poll

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Message-ID: <ed57b5fa-8b44-48de-904e-fe8da1939292@paulmck-laptop>
Date: Fri, 1 Mar 2024 14:29:19 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Yan Zhai <yan@...udflare.com>
Cc: 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,
	Joel Fernandes <joel@...lfernandes.org>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, mark.rutland@....com
Subject: Re: [PATCH v2] net: raise RCU qs after each threaded NAPI poll

On Fri, Mar 01, 2024 at 11:30:29AM -0600, Yan Zhai wrote:
> Hi Eric,
> 
> On Fri, Mar 1, 2024 at 2:30 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > 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?

I was thinking in terms of something like this (untested):

	#define rcu_softirq_qs_enable(enable_stmt, oldj) \
	do { \
		if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
		    time_after(oldj + HZ / 10, jiffies) { \
			rcu_softirq_qs(); \
			(oldj) = jiffies; \
		} \
		do  { enable_stmt; } while (0) \
	} while (0)

Then the call could be "rcu_softirq_qs_enable(local_bh_enable(), last_qs)",
where last_qs is initialized by the caller to jiffies.

The reason for putting "enable_stmt;" into anothor do-while loop is
in case someone typos an "else" as the first part of the "enable_stmt"
argument.

Would that work?

							Thanx, Paul