linux-kernel - Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

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Message-ID: <c971e3ef-384c-b96d-0a1b-e3bb42fee9ac@redhat.com>
Date:   Wed, 23 Oct 2019 12:47:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>
Cc:     Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <guro@...com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jann Horn <jannh@...gle.com>, Song Liu <songliubraving@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo

On 10/23/19 12:41 PM, Michal Hocko wrote:
> With a brown paper bag bug fixed. I have also added a note about low
> number of pages being more important as per Vlastimil's feedback
>
> From 0282f604144a5c06fdf3cf0bb2df532411e7f8c9 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.com>
> Date: Wed, 23 Oct 2019 12:13:02 +0200
> Subject: [PATCH] mm, vmstat: reduce zone->lock holding time by
>  /proc/pagetypeinfo
>
> 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.
>
> Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> Acked-by: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/vmstat.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4e885ecd44d1..c156ce24a322 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) {
> +					overflow = true;
> +					spin_unlock_irq(&zone->lock);
> +					cond_resched();
> +					spin_lock_irq(&zone->lock);
> +					break;
> +				}
> +			}
> +			seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
>  		}
>  		seq_putc(m, '\n');
>  	}

Reviewed-by: Waiman Long <longman@...hat.com>