linux-kernel - Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed

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Message-ID: <20150123143006.GC23123@twins.programming.kicks-ass.net>
Date:	Fri, 23 Jan 2015 15:30:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 08/12] time: Add warnings when overflows or underflows
 are observed

On Thu, Jan 22, 2015 at 04:09:23PM -0800, John Stultz wrote:
>  #ifdef CONFIG_DEBUG_TIMEKEEPING
> +#define WARNINGFREQ (HZ*300) /* 5 minute rate-limiting */
> +/*
> + * These simple flag variables are managed
> + * without locks, which is racy, but ok since
> + * we don't really care about being super
> + * precise about how many events were seen,
> + * just that a problem was observed.
> + */
> +static int timekeeping_underflow_seen;
> +static int timekeeping_overflow_seen;
> +
>  static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
>  {
>  
>  	cycle_t max_cycles = tk->tkr.clock->max_cycles;
>  	const char *name = tk->tkr.clock->name;
> +	static long last_warning; /* we always hold write on timekeeper lock */
>  
>  	if (offset > max_cycles)
>  		printk_deferred("ERROR: cycle offset (%lld) is larger then"
> @@ -133,28 +145,60 @@ static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
>  		printk_deferred("WARNING: cycle offset (%lld) is past"
>  			" the %s 50%% safety margin (%lld)\n",
>  			offset, name, max_cycles>>1);
> +
> +	if (timekeeping_underflow_seen) {
> +		if (jiffies - last_warning > WARNINGFREQ) {
> +			printk_deferred("WARNING: Clocksource underflow observed\n");
> +			last_warning = jiffies;
> +		}
> +		timekeeping_underflow_seen = 0;
> +	}
> +	if (timekeeping_overflow_seen) {
> +		if (jiffies - last_warning > WARNINGFREQ) {
> +			printk_deferred("WARNING: Clocksource overflow observed\n");
> +			last_warning = jiffies;
> +		}
> +		timekeeping_overflow_seen = 0;
> +	}
> +
>  }

Ah, ignore my last comment. Excellent!
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