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

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Date:   Wed, 30 Aug 2023 10:18:33 +0000
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 Wed, Aug 30, 2023 at 12:25:56PM +0800, Huacai Chen wrote:
> On Tue, Aug 29, 2023 at 10:46 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > 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!
>
> You can add my Tested-by, but Reported-by should be "Binbin Zhou
> <zhoubinbin@...ngson.cn>"

Thanks, if/when Paul takes it, he could kindly change the Reported-by, or I
could.

thanks,

 - Joel