On 10/23/19 6:27 AM, 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>
> ---
> 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
