[patch 064/192] mm: gup: allow FOLL_PIN to scale in SMP

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: mm: gup: allow FOLL_PIN to scale in SMP

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, below test is done on 40 cpus host with
Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz (must be with
CONFIG_GUP_TEST=y):

  $ sudo chrt -f 1 ./gup_test -a  -m 512 -j 40

Where we can get (average value for 40 threads):

  Old kernel: 477729.97 (+- 3.79%)
  New kernel:  89144.65 (+-11.76%)

On a similar condition with 256 cpus, this commits increases the SMP
scalability of pin_user_pages_fast() executed by different threads of the
same process by more than 4000%.

[peterx@xxxxxxxxxx: rewrite commit message, add parentheses against "(A & B)"]
Link: https://lkml.kernel.org/r/20210507150553.208763-3-peterx@xxxxxxxxxx
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Kirill Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/gup.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/gup.c~mm-gup-allow-foll_pin-to-scale-in-smp
+++ a/mm/gup.c
@@ -1320,7 +1320,7 @@ static __always_inline long __get_user_p
 		BUG_ON(*locked != 1);
 	}
 
-	if (flags & FOLL_PIN)
+	if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned))
 		atomic_set(&mm->has_pinned, 1);
 
 	/*
@@ -2641,7 +2641,7 @@ static int internal_get_user_pages_fast(
 				       FOLL_FAST_ONLY)))
 		return -EINVAL;
 
-	if (gup_flags & FOLL_PIN)
+	if ((gup_flags & FOLL_PIN) && !atomic_read(&current->mm->has_pinned))
 		atomic_set(&current->mm->has_pinned, 1);
 
 	if (!(gup_flags & FOLL_FAST_ONLY))
_