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Date:   Fri, 25 Oct 2019 14:08:50 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Michal Hocko <mhocko@kernel.org>
cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, Waiman Long <longman@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Jann Horn <jannh@google.com>, Song Liu <songliubraving@fb.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rafael Aquini <aquini@redhat.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo
In-Reply-To: <20191025072610.18526-3-mhocko@kernel.org>
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References: <20191025072610.18526-1-mhocko@kernel.org> <20191025072610.18526-3-mhocko@kernel.org>
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On Fri, 25 Oct 2019, Michal Hocko wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> pagetypeinfo_showfree_print is called by zone->lock held in irq mode.
> This is not really nice because it blocks both any interrupts on that
> cpu and the page allocator. On large machines this might even trigger
> the hard lockup detector.
> 
> Considering the pagetypeinfo is a debugging tool we do not really need
> exact numbers here. The primary reason to look at the outuput is to see
> how pageblocks are spread among different migratetypes and low number of
> pages is much more interesting therefore putting a bound on the number
> of pages on the free_list sounds like a reasonable tradeoff.
> 
> The new output will simply tell
> [...]
> Node    6, zone   Normal, type      Movable >100000 >100000 >100000 >100000  41019  31560  23996  10054   3229    983    648
> 
> instead of
> Node    6, zone   Normal, type      Movable 399568 294127 221558 102119  41019  31560  23996  10054   3229    983    648
> 
> The limit has been chosen arbitrary and it is a subject of a future
> change should there be a need for that.
> 
> While we are at it, also drop the zone lock after each free_list
> iteration which will help with the IRQ and page allocator responsiveness
> even further as the IRQ lock held time is always bound to those 100k
> pages.
> 
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

I think 100k is a very reasonable threshold.

Acked-by: David Rientjes <rientjes@google.com>

> ---
>  mm/vmstat.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4e885ecd44d1..ddb89f4e0486 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1383,12 +1383,29 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  			unsigned long freecount = 0;
>  			struct free_area *area;
>  			struct list_head *curr;
> +			bool overflow = false;
>  
>  			area = &(zone->free_area[order]);
>  
> -			list_for_each(curr, &area->free_list[mtype])
> -				freecount++;
> -			seq_printf(m, "%6lu ", freecount);
> +			list_for_each(curr, &area->free_list[mtype]) {
> +				/*
> +				 * Cap the free_list iteration because it might
> +				 * be really large and we are under a spinlock
> +				 * so a long time spent here could trigger a
> +				 * hard lockup detector. Anyway this is a
> +				 * debugging tool so knowing there is a handful
> +				 * of pages in this order should be more than
> +				 * sufficient
> +				 */
> +				if (++freecount >= 100000) {

I suppose it's most precise to check freecount > 1000000 to print >100000, 
but I doubt anybody cares :)

> +					overflow = true;
> +					break;
> +				}
> +			}
> +			seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
> +			spin_unlock_irq(&zone->lock);
> +			cond_resched();
> +			spin_lock_irq(&zone->lock);
>  		}
>  		seq_putc(m, '\n');
>  	}