linux-kernel - Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall

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Message-ID: <CAEXW_YRpT8wcLmsaHA6yMQ-ZCNYG7v4b4m-qvLHVxcbOJJcrPg@mail.gmail.com>
Date:   Tue, 29 Aug 2023 10:46:34 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     paulmck@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Z qiang <qiang.zhang1211@...il.com>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

On Tue, Aug 29, 2023 at 12:08 AM Huacai Chen <chenhuacai@...nel.org> wrote:
>
> Hi, Joel,
>
> On Tue, Aug 29, 2023 at 4:47 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > Hi Huacai,
> >
> > On Mon, Aug 28, 2023 at 11:13 AM Huacai Chen <chenhuacai@...nel.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