Re: [PATCH v8 03/17] mm: Assign id to every memcg-aware shrinker

From: Matthew Wilcox
Date: Tue Jul 03 2018 - 11:28:12 EST

On Tue, Jul 03, 2018 at 06:09:05PM +0300, Kirill Tkhai wrote:
> +++ b/mm/vmscan.c
> @@ -169,6 +169,49 @@ unsigned long vm_total_pages;
> static LIST_HEAD(shrinker_list);
> static DECLARE_RWSEM(shrinker_rwsem);
> +static DEFINE_IDR(shrinker_idr);
> +static int shrinker_nr_max;

So ... we've now got a list_head (shrinker_list) which contains all of
the shrinkers, plus a shrinker_idr which contains the memcg-aware shrinkers?

Why not replace the shrinker_list with the shrinker_idr? It's only used
twice in vmscan.c:

void register_shrinker_prepared(struct shrinker *shrinker)
list_add_tail(&shrinker->list, &shrinker_list);

list_for_each_entry(shrinker, &shrinker_list, list) {

The first is simply idr_alloc() and the second is

idr_for_each_entry(&shrinker_idr, shrinker, id) {

I understand there's a difference between allocating the shrinker's ID and
adding it to the list. You can do this by calling idr_alloc with NULL
as the pointer, and then using idr_replace() when you want to add the
shrinker to the list. idr_for_each_entry() skips over NULL entries.

This will actually reduce the size of each shrinker and be more
cache-efficient when calling the shrinkers. I think we can also get
rid of the shrinker_rwsem eventually, but let's leave it for now.