linux-kernel - Re: [PATCH 2/2] timer_list: convert timer list to be a proper seq

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Date:	Wed, 27 Mar 2013 12:14:12 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nathan Zimmer <nzimmer@....com>
Cc:	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH 2/2] timer_list: convert timer list to be a proper
 seq_file

On Tue, 26 Mar 2013 19:56:30 -0500 Nathan Zimmer <nzimmer@....com> wrote:

> When running with 4096 cores attemping to read /proc/timer_list will fail
> with an ENOMEM condition.  On a sufficantly large systems the total amount
> of data is more then 4mb, so it won't fit into a single buffer.  The
> failure can also occur on smaller systems when memory fragmentation is
> high as reported by Dave Jones.
> 
> Convert /proc/timer_list to a proper seq_file with its own iterator.  This
> is a little more complex given that we have to make two passes with two
> separate headers.
> 
> sysrq_timer_list_show also needed to be updated to reflect the fact that
> now timer_list_show only does one cpu at at time.
> 
> ...
>
> --- a/kernel/time/timer_list.c
> +++ b/kernel/time/timer_list.c
> @@ -20,6 +20,13 @@
>  
>  #include <asm/uaccess.h>
>  
> +
> +struct timer_list_iter {
> +	int cpu;
> +	bool second_pass;
> +	u64 now;
> +};
> +
>  typedef void (*print_fn_t)(struct seq_file *m, unsigned int *classes);
>  
>  DECLARE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases);
> @@ -247,43 +254,101 @@ static void timer_list_show_tickdevices_header(struct seq_file *m)
>  }
>  #endif
>  
> -static int timer_list_show(struct seq_file *m, void *v)
> +static inline void timer_list_header(struct seq_file *m, u64 now)

There's really no point in the explicit inline directive - modern gcc's
basically ignore it anyway.

>  {
> -	u64 now = ktime_to_ns(ktime_get());
> -	int cpu;
> -
>  	SEQ_printf(m, "Timer List Version: v0.7\n");
>  	SEQ_printf(m, "HRTIMER_MAX_CLOCK_BASES: %d\n", HRTIMER_MAX_CLOCK_BASES);
>  	SEQ_printf(m, "now at %Ld nsecs\n", (unsigned long long)now);
>  	SEQ_printf(m, "\n");
> +}
>
> ...
>
> +void sysrq_timer_list_show(void)
> +{
> +	u64 now = ktime_to_ns(ktime_get());
> +	int cpu;
> +
> +	timer_list_header(NULL, now);
>  
>  	for_each_online_cpu(cpu)

hm, it seems this code is taking the optimistic approach to CPU hotplug.

> -		print_cpu(m, cpu, now);
> +		print_cpu(NULL, cpu, now);

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