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Date:   Wed, 23 Oct 2019 22:02:56 +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: <20191023200256.GP17610@dhcp22.suse.cz>
References: <20191023102737.32274-3-mhocko@kernel.org>
 <20191023173423.12532-1-longman@redhat.com>
 <20191023180121.GN17610@dhcp22.suse.cz>
 <58a9adaf-9a1c-398b-dce1-cb30997807c1@redhat.com>
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On Wed 23-10-19 14:14:14, Waiman Long wrote:
> On 10/23/19 2:01 PM, Michal Hocko wrote:
> > 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.
> 
> If the free list is empty or is very short, there is probably no need to
> release and reacquire the lock. How about adding a check for a lower
> bound like:

Again, does it really make any sense to micro optimize something like
this. It is a debugging tool. I would rather go simple.
-- 
Michal Hocko
SUSE Labs