Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'

[Christophe JAILLET]

Le 03/11/2021 à 22:41, Matthew Wilcox a écrit :
> On Wed, Nov 03, 2021 at 10:35:34PM +0100, Christophe JAILLET wrote:
> > Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> > save a few cycles when it is known that the rcu lock is already
> > taken/released.
>
> If this is really important, we can add an __xa_load() which doesn't
> take the RCU read lock.

There are a few:
   rcu_read_lock();
   mem = xa_load(...);
   rcu_read_unlock();
patterns here and there.

I don't have any numbers of if saving some rcu_read_lock/rcu_read_unlock 
would be useful in these cases.

The only numbers I have are in [1].

[1]: 
https://lore.kernel.org/linux-kernel/cover.1634822969.git.asml.silence@xxxxxxxxx/

CJ


> I honestly think that the xarray is the wrong data structure here,
> and we'd be better off with a simple array of (start, pointer)
> tuples.
>
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> > ---
> >   mm/memremap.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 84de22c14567..012e8d23d365 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> >   	/* fall back to slow path lookup */
> >   	rcu_read_lock();
> >   	pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
> > -	if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
> > +	if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
> >   		pgmap = NULL;
> >   	rcu_read_unlock();
> >   
> > --
> > 2.30.2