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Date: Mon, 11 Mar 2024 16:55:37 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Yan Zhai <yan@cloudflare.com>
Cc: 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,
	Joel Fernandes <joel@joelfernandes.org>,
	Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= <toke@redhat.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>, mark.rutland@arm.com,
	Jesper Brouer <jesper@cloudflare.com>
Subject: Re: [PATCH v2] net: raise RCU qs after each threaded NAPI poll
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On Mon, Mar 11, 2024 at 05:58:16PM -0500, Yan Zhai wrote:
> On Fri, Mar 1, 2024 at 4:29 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > 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@google.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?
> >
> Thanks Paul, just got time to continue this thread as I was
> travelling. I think it is probably better to move
> preempt_disable/enable into the macro to avoid the friction. And also
> since this can affect NAPI thread, NAPI busy loop and XDP cpu map
> thread (+Jesper who reminded me about this), let me send a v3 later to
> cover all of those places.

OK, looking forward to seeing what you come up with.

							Thanx, Paul