linux-kernel - Re: [PATCH][RFC] PM / Hibernate: Feed NMI wathdog when creating snapshot

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Message-ID: <20170815160107.GA2541@yu-desktop-1.sh.intel.com>
Date:   Wed, 16 Aug 2017 00:01:08 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-pm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH][RFC] PM / Hibernate: Feed NMI wathdog when creating
 snapshot

Hi Michal,
On Tue, Aug 15, 2017 at 02:41:19PM +0200, Michal Hocko wrote:
> On Tue 15-08-17 01:19:16, Chen Yu wrote:
> [...]
> > @@ -2561,8 +2562,10 @@ 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++) {
> >  				swsusp_set_page_free(pfn_to_page(pfn + i));
> > +				touch_nmi_watchdog();
> > +			}
> 
> this is rather excessive. Why don't you simply call touch_nmi_watchdog
> once per every 1000 pages? Or once per free_list entry?
> 
Yes, this would be less costy, previously I thought that, since touch_nmi_watchdog()
would only update very limited amount of percpu variables and it is not costy when
comparing with the huge loop in radix tree searching by the swsusp_set_page_free(),
thus I put it in the deepest level in this loop, as this might be a safer place to avoid NMI.
> Moreover why don't you need to touch_nmi_watchdog in the loop over all
> pfns in the zone (right above this loop)?
As the NMI was triggered when checking the free_list rather than in the loop over
all pfns, it seems that the former has more possibility to catch a NMI if the
latter has already taken too much time. But yes, a safer way is to feed dog
in the latter too. I'll modify the code according to your suggestion.

Thanks,
	Yu
> -- 
> Michal Hocko
> SUSE Labs