linux-kernel - Re: [PATCH] perf: Prevent divide by zero exception in kernel/events/core.c

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Date:	Wed, 25 Sep 2013 14:12:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Knut Petersen <Knut_Petersen@...nline.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf: Prevent divide by zero exception in
 kernel/events/core.c

On Wed, Sep 25, 2013 at 02:07:35PM +0200, Knut Petersen wrote:
> From: Knut Petersen <Knut_Petersen@...nline.de>
> Date: Wed, 25 Sep 2013 13:34:57 +0200
> Subject: [PATCH] Enforce 1 as lower limit for perf_event_max_sample_rate
> 
> /proc/sys/kernel/perf_event_max_sample_rate will accept
> negative values as well as 0.
> 
> Negative values are unreasonable, and 0 causes a
> divide by zero exception in perf_proc_update_handler.
> 
> This patch enforces a lower limit of 1.
> 
> Signed-off-by: Knut Petersen <Knut_Petersen@...nline.de>
> ---
>  kernel/events/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index dd236b6..350bb53 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -198,6 +198,9 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
>  	if (ret || !write)
>  		return ret;
>  
> +	if (sysctl_perf_event_sample_rate <= 0)
> +		sysctl_perf_event_sample_rate = 1;
> +
>  	max_samples_per_tick = DIV_ROUND_UP(sysctl_perf_event_sample_rate, HZ);
>  	perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
>  	update_perf_cpu_limits();

The 'normal' way of doing that is changing the sysctl table entry to use
proc_dointvec_minmax and set .extra = &one. See for example
&sysctl_sched_cfs_bandwidth_slice in kernel/sysctl.c
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