Re: [patch 13/14] dentries: Extract common code to remove dentry from lru

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

On Wed, 24 Oct 2007 19:50:19 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:

> On Wed, 24 Oct 2007, Andrew Morton wrote:
> 
> > > Sometimes we check the list head using list_empty() so we cannot avoid 
> > > list_del_init. Always using list_del_init results in a consistent state of 
> > > affairs before the object is freed (which the slab defrag patchset depends 
> > > on)
> > 
> > OK, but it's slower.
> > 
> > So I think it should be changlogged as such, with an explanation that there
> > will (hopefully) be a net benefit because it enables slab defrag, and it
> > should be moved into the slab-defrag patchset.
> 
> really?
> 
> list_del_init does:
> 
> static inline void list_del_init(struct list_head *entry)
> {
>         __list_del(entry->prev, entry->next);
>         INIT_LIST_HEAD(entry);
> }
> 
> So it touches the cachelines of the entry and prev/next to fix up the 
> links.
> 
> list_del does:
> 
> #ifndef CONFIG_DEBUG_LIST
> static inline void list_del(struct list_head *entry)
> {
>         __list_del(entry->prev, entry->next);
>         entry->next = LIST_POISON1;
>         entry->prev = LIST_POISON2;
> }
> #else
> extern void list_del(struct list_head *entry);
> #endif
> 
> In the !DEBUG case it touches the same cachelines. The only change is that 
> we poison entry. 

bugger, I'd forgotten that we do the poisoning even if !CONFIG_DEBUG_LIST.

We really shouldn't do that, especially now that we have the list debugging
config option.

Fixing this might break net/rxrpc/af_rxrpc.c and detach_timer(), but they
deserve to be broken.

> So its not slower.

It should be.

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