On 5/6/21 4:25 PM, Peter Xu wrote:
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
has_pinned cannot be written by each pin-fast or it won't scale in
SMP. This isn't "false sharing" strictly speaking (it's more like
"true non-sharing"), but it creates the same SMP scalability
bottleneck of "false sharing".
To verify the improvement a new "pin_fast.c" program was added to
the will-it-scale benchmark.
...
This commits increases the SMP scalability of pin_user_pages_fast()
executed by different threads of the same process by more than 4000%.
Remarkable! I mean, yes, everyone knows that atomic writes are
"expensive", but this is a fun, dramatic example of just *how*
expensive they can get, once you start doing contended atomic writes.
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Other notes, that don't have any effect on the above reviewed-by
tag:
On the commit log, I will add a "+1" to the idea of deleting the
pin_fast.c contents from the commit log, and just providing a URL
instead. No need to put C programs in the commit log, IMHO, especially
when you have them elsewhere anyway.
thanks,
--
John Hubbard
NVIDIA
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/gup.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 63a079e361a3d..8b513e1723b45 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1292,7 +1292,7 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
BUG_ON(*locked != 1);
}
- if (flags & FOLL_PIN)
+ if (flags & FOLL_PIN && !atomic_read(&mm->has_pinned))
atomic_set(&mm->has_pinned, 1);
/*
@@ -2617,7 +2617,7 @@ static int internal_get_user_pages_fast(unsigned long start,
FOLL_FAST_ONLY)))
return -EINVAL;
- if (gup_flags & FOLL_PIN)
+ if (gup_flags & FOLL_PIN && !atomic_read(¤t->mm->has_pinned))
atomic_set(¤t->mm->has_pinned, 1);
if (!(gup_flags & FOLL_FAST_ONLY))
thanks,
--
John Hubbard
NVIDIA
