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:	Mon, 05 Jan 2015 17:07:30 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Russell King <linux@....linux.org.uk>,
	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 05/01/15 15:19, 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;
>> +};
>> +
>> +/* Safe printing in NMI context */
>> +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
>> +
>> +/* "in progress" flag of arch_trigger_all_cpu_backtrace */
>> +static unsigned long backtrace_flag;
>> +
>> +/*
>> + * It is not safe to call printk() directly from NMI handlers.
>> + * It may be fine if the NMI detected a lock up and we have no choice
>> + * but to do so, but doing a NMI on all other CPUs to get a back trace
>> + * can be done with a sysrq-l. We don't want that to lock up, which
>> + * can happen if the NMI interrupts a printk in progress.
>> + *
>> + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
>> + * the content into a per cpu seq_buf buffer. Then when the NMIs are
>> + * all done, we can safely dump the contents of the seq_buf to a printk()
>> + * from a non NMI context.
>> + */
>> +static int nmi_vprintk(const char *fmt, va_list args)
>> +{
>> +	struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
>> +	unsigned int len = seq_buf_used(&s->seq);
>> +
>> +	seq_buf_vprintf(&s->seq, fmt, args);
>> +	return seq_buf_used(&s->seq) - len;
>> +}
>> +
> 
> 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.

Sounds like a good idea. I'll take a look at this.


Daniel.
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