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@xxxxxxxxxx>
> >> ---
> >> 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
