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To:     Huacai Chen <chenhuacai@loongson.cn>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        chenhuacai@kernel.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, Huacai Chen <chenhuacai@loongson.cn>,
        stable@vger.kernel.org, Binbin Zhou <zhoubinbin@loongson.cn>
Subject: Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()
In-Reply-To: <20230822040248.329442-1-chenhuacai@loongson.cn>
References: <20230822040248.329442-1-chenhuacai@loongson.cn>
Date:   Thu, 24 Aug 2023 11:37:41 +0200
Message-ID: <87lee0dbmi.ffs@tglx>
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List-ID: <stable.vger.kernel.org>
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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