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
