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

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Message-ID: <20200211222754.0185b9b3@rorschach.local.home>
Date:   Tue, 11 Feb 2020 22:27:54 -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 12:04:03 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> Oh, yes, ftrace printks a lot. But I sort of forgot why don't we do
> the same for "regular" NMIs. So NMIs use per-cpu buffers, expect for
> NMIs which involve ftrace dump. I'm missing something here.

Well, ftrace_dump() is called from NMI context when the system is about
to crash (ftrace_dump_on_oops). And at that moment, we care more about
the trace than other output (it's most like to contain the information
that caused the bug).

But for things like sysrq-l, that does a printk in NMI context for
normal operations, we don't want strange races to occur and affect
other printk operations. Having them in buffers controls the output a
bit better.

But with the new printk ring buffer work, perhaps that's no longer
necessary.

-- Steve