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

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Message-ID: <a3510617-fd23-9f90-3c40-700bcb0f353c@redhat.com>
Date:   Wed, 23 Oct 2019 10:56:30 -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>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo

On 10/23/19 6:27 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.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 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>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/vmstat.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4e885ecd44d1..762034fc3b83 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1386,8 +1386,25 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  
>  			area = &(zone->free_area[order]);
>  
> -			list_for_each(curr, &area->free_list[mtype])
> +			list_for_each(curr, &area->free_list[mtype]) {
>  				freecount++;
> +				/*
> +				 * 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) {
> +					seq_printf(m, ">%6lu ", freecount);
> +					spin_unlock_irq(&zone->lock);
> +					cond_resched();
> +					spin_lock_irq(&zone->lock);
> +					continue;
list_for_each() is a for loop. The continue statement will just iterate
the rests with the possibility that curr will be stale. Should we use
goto to jump after the seq_print() below?
> +				}
> +			}
>  			seq_printf(m, "%6lu ", freecount);
>  		}
>  		seq_putc(m, '\n');

Cheers,
Longman