From: Andrew Morton <akpm@linux-foundation.org>
To: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Petr Mladek <pmladek@suse.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Subject: Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected
Date: Mon, 10 Feb 2020 14:51:18 -0800 [thread overview]
Message-ID: <20200210145118.1d80e248c9206aeafd5baae6@linux-foundation.org> (raw)
In-Reply-To: <158132813726.1980.17382047082627699898.stgit@buzz>
On Mon, 10 Feb 2020 12:48:57 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
> In NMI context printk() could save messages into per-cpu buffers and
> schedule flush by irq_work when IRQ are unblocked. This means message
> about hardlockup appears in kernel log only when/if lockup is gone.
I think I understand what this means. The hard lockup detector runs at
NMI time but if it detects a lockup within IRQ context it cannot call
printk, because it's within NMI context, where synchronous printk
doesn't work. Yes?
> Comment in irq_work_queue_on() states that remote IPI aren't NMI safe
> thus printk() cannot schedule flush work to another cpu.
>
> This patch adds simple atomic counter of detected hardlockups and
> flushes all per-cpu printk buffers in context softlockup watchdog
> at any other cpu when it sees changes of this counter.
And I think this works because the softlockup detector runs within irq
context?
>
> ...
>
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -92,6 +92,26 @@ static int __init hardlockup_all_cpu_backtrace_setup(char *str)
> }
> __setup("hardlockup_all_cpu_backtrace=", hardlockup_all_cpu_backtrace_setup);
> # endif /* CONFIG_SMP */
> +
> +atomic_t hardlockup_detected = ATOMIC_INIT(0);
> +
> +static inline void flush_hardlockup_messages(void)
I don't think this needs to be inlined?
> +{
> + static atomic_t flushed = ATOMIC_INIT(0);
> +
> + /* flush messages from hard lockup detector */
> + if (atomic_read(&hardlockup_detected) != atomic_read(&flushed)) {
> + atomic_set(&flushed, atomic_read(&hardlockup_detected));
> + printk_safe_flush();
> + }
> +}
Could we add some explanatory comments here? Explain to the reader why
this code exists, what purpose it serves? Basically a micro version of
the above changelog.
>
> ...
>
next prev parent reply other threads:[~2020-02-10 22:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-10 9:48 [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected Konstantin Khlebnikov
2020-02-10 22:51 ` Andrew Morton [this message]
2020-02-11 11:01 ` Konstantin Khlebnikov
2020-02-11 8:14 ` Kirill Tkhai
2020-02-11 12:36 ` Konstantin Khlebnikov
2020-02-12 14:54 ` Petr Mladek
2020-02-13 13:05 ` Konstantin Khlebnikov
2020-02-12 1:15 ` Sergey Senozhatsky
2020-02-12 2:49 ` Steven Rostedt
2020-02-12 3:04 ` Sergey Senozhatsky
2020-02-12 3:27 ` Steven Rostedt
2020-02-12 14:18 ` Petr Mladek
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