linux-kernel - Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when hardlockup detected

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Date:   Tue, 11 Feb 2020 21:49:58 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: [PATCH] kernel/watchdog: flush all printk nmi buffers when
 hardlockup detected

On Wed, 12 Feb 2020 10:15:51 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> 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?

Could it be because its from ftrace_dump() which can spit out millions
of lines from NMI context?

-- Steve