Re: [RFC][PATCHv3 7/7] zsmalloc: register a shrinker to trigger auto-compaction

[Minchan Kim <minchan@kernel.org>]

Hi Sergey,

Sorry for too late reply.

On Mon, Jun 29, 2015 at 05:57:44PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> thanks for review.
> 
> On (06/29/15 16:07), Minchan Kim wrote:
> [..]
> > >  			if (!migrate_zspage(pool, class, &cc))
> > > -				break;
> > > +				goto out;
> > 
> > It should retry with another target_page instead of going out.
> 
> yep.
> 
> [..]
> > > +static unsigned long zs_shrinker_scan(struct shrinker *shrinker,
> > > +		struct shrink_control *sc)
> > > +{
> [..]
> > 
> > Returns migrated object count.
> > 
> [..]
> > > +static unsigned long zs_shrinker_count(struct shrinker *shrinker,
> > > +		struct shrink_control *sc)
> > > +{
> [..]
> > 
> > But it returns wasted_obj / max_obj_per_zspage?
> > 
> 
> Good catch.
> So,  __zs_compact() and zs_shrinker_count() are ok. Returning
> "wasted_obj / max_obj_per_zspage" from zs_can_compact() makes
> sense there. The only place is zs_shrinker_scan()->zs_compact().

I want to make zs_can_compact return freeable page unit.

ie,

        return obj_wasted * class->pages_per_zspage;
  
and let's make __zs_compact returns the number of freed pages.

IOW, I like your (c).

> 
> Hm, I can think of:
> 
> (a) We can change zs_compact() to return the total number of
> freed zspages. That will not really change a user visible
> interface. We export (fwiw) the number of compacted objects
> in mm_stat. Basically, this is internal zsmalloc() counter and
> no user space program can ever do anything with that data. From
> that prospective we will just replace one senseless number with
> another (equally meaningless) one.
> 
> 
> (b) replace zs_compact() call in zs_shrinker_scan() with a class loop
> 
> 1764         int i;
> 1765         unsigned long nr_migrated = 0;
> 1766         struct size_class *class;
> 1767
> 1768         for (i = zs_size_classes - 1; i >= 0; i--) {
> 1769                 class = pool->size_class[i];
> 1770                 if (!class)
> 1771                         continue;
> 1772                 if (class->index != i)
> 1773                         continue;
> 1774                 nr_migrated += __zs_compact(pool, class);
> 1775         }
> 1776
> 1777         return nr_migrated;
> 
> But on every iteration increment nr_migrated with
> 		"nr_migrated += just_migrated / max_obj_per_zspage"
> 
> (which will be unnecessary if zs_compact() will return the number of freed
> zspages).
> 
> So, (b) is mostly fine, except that we already have several pool->size_class
> loops, with same `if (!class)' and `if (class->index...)' checks; and it
> asks for some sort of refactoring or... a tricky for_each_class() define.
> 
> 
> In both cases, however, we don't tell anything valuable to user space.
> Thus,
> 
> (c) Return from zs_compact() the number of pages (PAGE_SIZE) freed.
> And change compaction to operate in terms of pages (PAGE_SIZE). At
> least mm_stat::compacted will turn into something useful for user
> space.

Yes.

Thanks.

> 
> 	-ss

-- 
Kind regards,
Minchan Kim

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