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Date:   Wed, 23 Oct 2019 20:01:21 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        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>
Subject: Re: [PATCH 1/2] mm, vmstat: Release zone lock more frequently when
 reading /proc/pagetypeinfo
Message-ID: <20191023180121.GN17610@dhcp22.suse.cz>
References: <20191023102737.32274-3-mhocko@kernel.org>
 <20191023173423.12532-1-longman@redhat.com>
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On Wed 23-10-19 13:34:22, Waiman Long wrote:
> With a threshold of 100000, it is still possible that the zone lock
> will be held for a very long time in the worst case scenario where all
> the counts are just below the threshold. With up to 6 migration types
> and 11 orders, it means up to 6.6 millions.
> 
> Track the total number of list iterations done since the acquisition
> of the zone lock and release it whenever 100000 iterations or more have
> been completed. This will cap the lock hold time to no more than 200,000
> list iterations.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/vmstat.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 57ba091e5460..c5b82fdf54af 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1373,6 +1373,7 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  					pg_data_t *pgdat, struct zone *zone)
>  {
>  	int order, mtype;
> +	unsigned long iteration_count = 0;
>  
>  	for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
>  		seq_printf(m, "Node %4d, zone %8s, type %12s ",
> @@ -1397,15 +1398,24 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  				 * of pages in this order should be more than
>  				 * sufficient
>  				 */
> -				if (++freecount >= 100000) {
> +				if (++freecount > 100000) {
>  					overflow = true;
> -					spin_unlock_irq(&zone->lock);
> -					cond_resched();
> -					spin_lock_irq(&zone->lock);
> +					freecount--;
>  					break;
>  				}
>  			}
>  			seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
> +			/*
> +			 * Take a break and release the zone lock when
> +			 * 100000 or more entries have been iterated.
> +			 */
> +			iteration_count += freecount;
> +			if (iteration_count >= 100000) {
> +				iteration_count = 0;
> +				spin_unlock_irq(&zone->lock);
> +				cond_resched();
> +				spin_lock_irq(&zone->lock);
> +			}

Aren't you overengineering this a bit? If you are still worried then we
can simply cond_resched for each order
diff --git a/mm/vmstat.c b/mm/vmstat.c
index c156ce24a322..ddb89f4e0486 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1399,13 +1399,13 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
 				 */
 				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);
+			spin_unlock_irq(&zone->lock);
+			cond_resched();
+			spin_lock_irq(&zone->lock);
 		}
 		seq_putc(m, '\n');
 	}

I do not have a strong opinion here but I can fold this into my patch 2.
-- 
Michal Hocko
SUSE Labs