On Wed, Oct 23, 2019 at 03:32:05PM +0200, Vlastimil Babka wrote:
> On 10/23/19 12:27 PM, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@xxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>
> Hmm dunno, I would rather e.g. hide the file behind some config or boot
> option than do this. Or move it to /sys/kernel/debug ?
>
You beat me to it. I was going to suggest moving it to debug, as well.
> > ---
> > 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;
> > + }
> > + }
> > seq_printf(m, "%6lu ", freecount);
> > }
> > seq_putc(m, '\n');
> >
>
