linux-kernel - Re: [PATCH 3.19-rc2 v13 4/5] ARM: Add support for on-demand backtrace of other CPUs

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Date:	Fri, 9 Jan 2015 16:48:01 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH 3.19-rc2 v13 4/5] ARM: Add support for on-demand
 backtrace of other CPUs

On Mon, Jan 05, 2015 at 10:19:25AM -0500, Steven Rostedt wrote:
> On Mon,  5 Jan 2015 14:54:58 +0000
> Daniel Thompson <daniel.thompson@...aro.org> wrote:
> > +/* For reliability, we're prepared to waste bits here. */
> > +static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
> > +static  cpumask_t printtrace_mask;
> > +
> > +#define NMI_BUF_SIZE		4096
> > +
> > +struct nmi_seq_buf {
> > +	unsigned char		buffer[NMI_BUF_SIZE];
> > +	struct seq_buf		seq;
> > +};

Am I missing something or does this limit us to 4096 characters of
backtrace output per CPU?

> This is the same code as in x86. I wonder if we should move the
> duplicate code into kernel/printk/ and have it compiled if the arch
> requests it (CONFIG_ARCH_WANT_NMI_PRINTK or something). That way we
> don't have 20 copies of the same nmi_vprintk() and later find that we
> need to change it, and have to change it in 20 different archs.

Agreed, though I wonder about the buffer size.

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