Re: [PATCH] mm: Use special value SHRINKER_REGISTERING instead list_empty() check

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 03 Aug 2018 18:36:14 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:

> The patch introduces a special value SHRINKER_REGISTERING to use instead
> of list_empty() to detect a semi-registered shrinker.
> 
> This should be clearer for a reader since "list is empty"  is not
> an intuitive state of a shrinker), and this gives a better assembler
> code:
> 
> Before:
> callq  <idr_find>
> mov    %rax,%r15
> test   %rax,%rax
> je     <shrink_slab_memcg+0x1d5>
> mov    0x20(%rax),%rax
> lea    0x20(%r15),%rdx
> cmp    %rax,%rdx
> je     <shrink_slab_memcg+0xbd>
> mov    0x8(%rsp),%edx
> mov    %r15,%rsi
> lea    0x10(%rsp),%rdi
> callq  <do_shrink_slab>
> 
> After:
> callq  <idr_find>
> mov    %rax,%r15
> lea    -0x1(%rax),%rax
> cmp    $0xfffffffffffffffd,%rax
> ja     <shrink_slab_memcg+0x1cd>
> mov    0x8(%rsp),%edx
> mov    %r15,%rsi
> lea    0x10(%rsp),%rdi
> callq  ffffffff810cefd0 <do_shrink_slab>
> 
> Also, improve the comment.

All this isn't terribly nice.  Why can't we avoid installing the
shrinker into the idr until it is fully initialized?

Or extend the down_write(shrinker_rwsem) coverage so it protects the
entire initialization, instead of only in the prealloc_memcg_shrinker()
part of that initialization.  This is not as good - it would be better
to do all the initialization locklessly then just install the fully
initialized thing under the lock.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -170,6 +170,21 @@ static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
>  #ifdef CONFIG_MEMCG_KMEM
> +
> +/*
> + * There is a window between prealloc_shrinker()
> + * and register_shrinker_prepared(). We don't want
> + * to clear bit of a shrinker in such the state
> + * in shrink_slab_memcg(), since this will impose
> + * restrictions on a code registering a shrinker
> + * (they would have to guarantee, their LRU lists
> + * are empty till shrinker is completely registered).
> + * So, we use this value to detect the situation,
> + * when id is assigned, but shrinker is not completely
> + * registered yet.
> + */

This comment is still quite hard to understand.  Could you please spend
a little more time over it?