Re: [PATCH 2/2] mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

From: Michal Hocko
Date: Fri Oct 25 2019 - 07:43:56 EST

On Fri 25-10-19 07:18:37, Qian Cai wrote:
> ï
> > On Oct 25, 2019, at 3:26 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > 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 and low number of
> > pages is much more interesting 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
> It was mentioned that developers could use this file is to see the
> movement of those numbers for debugging, so this supposed to introduce
> regressions as there is no movement anymore for those 100k+ items?

Can you provide an explicit example please? As the changelog mentions
it is the "low numbers" that is really interesting when debugging
fragmentation issues. Because we are running out of a respective migrate
type and it is interesting to see why and what compaction is doing etc.

Having more than 100k pages on the respective migrate type list is a
good sign that the migrate type is not in problems. Comparing multiple
migrate types with >100k pages doesn't really need a large precision

Now, I do agree that some debugging tools might get confused by > in the
output but can we wait for reports and see whether anybody actually
cares? It is more likely that fixing one off debugging tools wouldn't be
a big deal.

Michal Hocko