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

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Date:   Mon, 28 Aug 2023 16:47:07 -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()

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.

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!

thanks,

 - Joel