From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>,
Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected
Date: Wed, 12 Feb 2020 15:18:32 +0100 [thread overview]
Message-ID: <20200212141832.lqmzzdi77hb6yrhu@pathway.suse.cz> (raw)
In-Reply-To: <20200212011551.GA13208@google.com>
On Wed 2020-02-12 10:15:51, Sergey Senozhatsky wrote:
> On (20/02/10 12:48), Konstantin Khlebnikov 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.
> >
> > 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.
>
> Petr, could you remind me, why do we do PRINTK_NMI_DIRECT_CONTEXT_MASK
> only from ftrace?
There was a possible deadlock when printing backtraces from all CPUs.
The CPUs were serialized via a lock in nmi_cpu_backtrace(). One of
them might have been interrupted under logbuf_lock.
ftrace was needed because it printed too many messages. And it was
safe because the ftrace log was read from a single CPU without
any lock.
Best Regards,
Petr
prev parent reply other threads:[~2020-02-12 14:18 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
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 [this message]
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