linux-kernel - Re: [PATCH v8 1/4] nmi_backtrace: add more trigger_*_cpu

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Message-ID: <20160818141242.GK26194@pathway.suse.cz>
Date:	Thu, 18 Aug 2016 16:12:42 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Andrew Morton <akpm@...l.org>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	"Ralf Baechle ralf @ linux-mips . org David S. Miller" 
	<davem@...emloft.net>, linux-mips@...ux-mips.org,
	sparclinux@...r.kernel.org, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 1/4] nmi_backtrace: add more trigger_*_cpu_backtrace()
 methods

On Tue 2016-08-16 15:50:21, Chris Metcalf wrote:
> Currently you can only request a backtrace of either all cpus, or
> all cpus but yourself.  It can also be helpful to request a remote
> backtrace of a single cpu, and since we want that, the logical
> extension is to support a cpumask as the underlying primitive.
> 
> This change modifies the existing lib/nmi_backtrace.c code to take
> a cpumask as its basic primitive, and modifies the linux/nmi.h code
> to use the new "cpumask" method instead.
> 
> The mips code ignored the "include_self" boolean but with this change
> it will now also dump a local backtrace if requested.
> 
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index 7429ad09fbe3..fea1fa7726e3 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -569,9 +569,16 @@ static void arch_dump_stack(void *info)
>  	dump_stack();
>  }
>  
> -void arch_trigger_all_cpu_backtrace(bool include_self)
> +void arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool exclude_self)
>  {
> -	smp_call_function(arch_dump_stack, NULL, 1);
> +	long this_cpu = get_cpu();
> +
> +	if (cpumask_test_cpu(this_cpu, mask) && !exclude_self)
> +		dump_stack();

The bit is not cleared in the mask. Therefore arch_dump_stack
will get called for this CPU as well.

We could either use similar tricks as in lib/nmi_backtrace().
I mean to copy the mask into a global variable and prevent
parallel call with a backtrace_flag.

Or we could ignore the exclude_self flag as it was done
before. It is a separate problem after all and it need
not be solved in this patch(set).

> +
> +	smp_call_function_many(mask, arch_dump_stack, NULL, 1);
> +
> +	put_cpu();
>  }

Otherwise the patch patch looks good to me.

Best Regards,
Petr