Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

From: Joel Fernandes <joel@joelfernandes.org>
To: Huacai Chen <chenhuacai@kernel.org>
Cc: paulmck@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Z qiang <qiang.zhang1211@gmail.com>,
	Huacai Chen <chenhuacai@loongson.cn>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@kernel.org>,
	John Stultz <jstultz@google.com>, Stephen Boyd <sboyd@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org, Binbin Zhou <zhoubinbin@loongson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()
Date: Tue, 29 Aug 2023 10:46:34 -0400	[thread overview]
Message-ID: <CAEXW_YRpT8wcLmsaHA6yMQ-ZCNYG7v4b4m-qvLHVxcbOJJcrPg@mail.gmail.com> (raw)
In-Reply-To: <CAAhV-H6oN69rV2OyGzUganRv4KbS7a3_gNyWhCqVp51Ay9Q_=g@mail.gmail.com>

On Tue, Aug 29, 2023 at 12:08 AM Huacai Chen <chenhuacai@kernel.org> wrote:
>
> Hi, Joel,
>
> On Tue, Aug 29, 2023 at 4:47 AM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > Hi Huacai,
> >
> > On Mon, Aug 28, 2023 at 11:13 AM Huacai Chen <chenhuacai@kernel.org> wrote:
> > >
> > [...]
> > > >
> > > > > [Huacai]
> > > > > I also think the original patch should be OK, but I have another
> > > > > question: what will happen if the current GP ends before
> > > > > nr_fqs_jiffies_stall reaches zero?
> > > >
> > > > Nothing should happen. Stall detection only happens when a GP is in
> > > > progress. If a new GP starts, it resets nr_fqs_jiffies_stall.
> > > >
> > > > Or can you elaborate your concern more?
> > > OK, I will test your patch these days. Maybe putting
> > > nr_fqs_jiffies_stall before jiffies_force_qs is better, because I
> > > think putting an 'int' between two 'long' is wasting space. :)
> >
> > That's a good point and I'll look into that.
> Another point, is it better to replace ULONG_MAX with ULONG_MAX/4 as
> Paul suggested?
>

I could do that but I don't feel too strongly about it. I will keep it
at ULONG_MAX if it's OK with everyone.

> > Meanwhile I pushed the patch out to my 6.4 stable tree for testing on my fleet.
> >
> > Ideally, I'd like to change the stall detection test in the rcutorture
> > to actually fail rcutorture if stalls don't happen in time. But at
> > least I verified this manually using rcutorture.
> >
> > I should also add a documentation patch for stallwarn.rst to document
> > the understandable sensitivity of RCU stall detection to jiffies
> > updates (or lack thereof). Or if you have time, I'd appreciate support
> > on such a patch (not mandatory but I thought it would not hurt to
> > ask).
> >
> > Looking forward to how your testing goes as well!
> I have tested, it works for KGDB.

Thanks! If you don't mind, I will add your Tested-by tag to the patch
and send it out soon. My tests also look good!

 - Joel