linux-kernel - Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall

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Date:   Thu, 24 Aug 2023 11:37:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        chenhuacai@...nel.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, Huacai Chen <chenhuacai@...ngson.cn>,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

On Tue, Aug 22 2023 at 12:02, Huacai Chen wrote:
> + * This function may be called in NMI context, so we cannot use ktime_get_ns()
> + * and ktime_get_coarse_ns(). Instead, we use their inaccurate but safe friends
> + * ktime_get_mono_fast_ns() and ktime_get_seconds() which will cause rcu_state.
> + * jiffies_stall to be a little large than expected (harmless and safer).

What's inaccurate about ktime_get_mono_fast_ns()? Bogus comments are
even worse than no comments.

>   */
>  void rcu_cpu_stall_reset(void)
>  {
> +	u64 curr, last, delta;
> +
> +	curr = ktime_get_mono_fast_ns();
> +	last = ktime_get_seconds() * NSEC_PER_SEC;

So this will trigger a warning when someone debugs suspend with KGDB.

It seems the approach taken here seems to be to throw stuff at the wall
and see what sticks.

Thanks,

        tglx