Re: [PATCH] PM / Hibernate: Feed the wathdog when creating snapshot

[Andrew Morton]

On Mon, 21 Aug 2017 23:08:18 +0800 Chen Yu <yu.c.chen@xxxxxxxxx> wrote:

> There is a problem that when counting the pages for creating
> the hibernation snapshot will take significant amount of
> time, especially on system with large memory. Since the counting
> job is performed with irq disabled, this might lead to NMI lockup.
> The following warning were found on a system with 1.5TB DRAM:
> 
> ...
> 
> It has taken nearly 20 seconds(2.10GHz CPU) thus the NMI lockup
> was triggered. In case the timeout of the NMI watch dog has been
> set to 1 second, a safe interval should be 6590003/20 = 320k pages
> in theory. However there might also be some platforms running at a
> lower frequency, so feed the watchdog every 100k pages.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2531,9 +2532,12 @@ void drain_all_pages(struct zone *zone)
>  
>  #ifdef CONFIG_HIBERNATION
>  
> +/* Touch watchdog for every WD_INTERVAL_PAGE pages. */
> +#define WD_INTERVAL_PAGE	(100*1024)
> +
>  void mark_free_pages(struct zone *zone)
>  {
> -	unsigned long pfn, max_zone_pfn;
> +	unsigned long pfn, max_zone_pfn, page_num = 0;
>  	unsigned long flags;
>  	unsigned int order, t;
>  	struct page *page;
> @@ -2548,6 +2552,9 @@ void mark_free_pages(struct zone *zone)
>  		if (pfn_valid(pfn)) {
>  			page = pfn_to_page(pfn);
>  
> +			if (!((page_num++) % WD_INTERVAL_PAGE))
> +				touch_nmi_watchdog();
> +
>  			if (page_zone(page) != zone)
>  				continue;
>  
> @@ -2561,8 +2568,11 @@ void mark_free_pages(struct zone *zone)
>  			unsigned long i;
>  
>  			pfn = page_to_pfn(page);
> -			for (i = 0; i < (1UL << order); i++)
> +			for (i = 0; i < (1UL << order); i++) {
> +				if (!((page_num++) % WD_INTERVAL_PAGE))
> +					touch_nmi_watchdog();
>  				swsusp_set_page_free(pfn_to_page(pfn + i));
> +			}
>  		}
>  	}
>  	spin_unlock_irqrestore(&zone->lock, flags);

hm, is it really worth all the WD_INTERVAL_PAGE stuff? 
touch_nmi_watchdog() is pretty efficient and calling it once-per-page
may not have a measurable effect.

And if we're really concerned about the performance impact it would be
better to make WD_INTERVAL_PAGE a power of 2 (128*1024?) to avoid the
modulus operation.