linux-kernel - Re: [tip:perf/core] x86/nmi: Push duration printk() to irq context

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Date:	Mon, 10 Feb 2014 08:45:16 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org, tglx@...utronix.de, dzickus@...hat.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] x86/nmi: Push duration printk() to irq context

On 02/10/2014 05:29 AM, tip-bot for Peter Zijlstra wrote:
> x86/nmi: Push duration printk() to irq context
> 
> Calling printk() from NMI context is bad (TM), so move it to IRQ
> context.

Bad since the I/O device that we're doing it to may be slow and make the
NMI painfully long?

I can see why it might be a bad idea, but I'm unsold that it is
*universally* a bad idea.

> In doing so we slightly change (probably wreck) the debugfs
> nmi_longest_ns thingy, in that it doesn't update to reflect the
> longest, nor does writing to it reset the count.

The reason I coded this up was that NMIs were firing off so fast that
nothing else was getting a chance to run.  With this patch, at least the
printk() would come out and I'd have some idea what was going on.


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