Hi Peter, Andrea,
On Fri, May 7, 2021 at 7:26 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>
> has_pinned 32bit can be packed in the MMF_HAS_PINNED bit as a noop
> cleanup.
>
> Any atomic_inc/dec to the mm cacheline shared by all threads in
> pin-fast would reintroduce a loss of SMP scalability to pin-fast, so
> there's no future potential usefulness to keep an atomic in the mm for
> this.
>
> set_bit(MMF_HAS_PINNED) will be theoretically a bit slower than
> WRITE_ONCE (atomic_set is equivalent to WRITE_ONCE), but the set_bit
> (just like atomic_set after this commit) has to be still issued only
> once per "mm", so the difference between the two will be lost in the
> noise.
>
> will-it-scale "mmap2" shows no change in performance with enterprise
> config as expected.
>
> will-it-scale "pin_fast" retains the > 4000% SMP scalability
> performance improvement against upstream as expected.
>
> This is a noop as far as overall performance and SMP scalability are
> concerned.
>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> [peterx: Fix build for task_mmu.c, introduce mm_set_has_pinned_flag, fix
> comment here and there]
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Thanks for your patch, which is now in linux-next.
> diff --git a/mm/gup.c b/mm/gup.c
> index 9933bc5c2eff2..bb130723a6717 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1270,6 +1270,17 @@ int fixup_user_fault(struct mm_struct *mm,
> }
> EXPORT_SYMBOL_GPL(fixup_user_fault);
>
> +/*
> + * Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
> + * lifecycle. Avoid setting the bit unless necessary, or it might cause write
> + * cache bouncing on large SMP machines for concurrent pinned gups.
> + */
> +static inline void mm_set_has_pinned_flag(unsigned long *mm_flags)
> +{
> + if (!test_bit(MMF_HAS_PINNED, mm_flags))
> + set_bit(MMF_HAS_PINNED, mm_flags);
> +}
> +
> /*
> * Please note that this function, unlike __get_user_pages will not
> * return 0 for nr_pages > 0 without FOLL_NOWAIT
> @@ -1292,8 +1303,8 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
> BUG_ON(*locked != 1);
> }
>
> - if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned))
> - atomic_set(&mm->has_pinned, 1);
> + if (flags & FOLL_PIN)
> + mm_set_has_pinned_flag(&mm->flags);
>
> /*
> * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
> @@ -2617,8 +2628,8 @@ static int internal_get_user_pages_fast(unsigned long start,
> FOLL_FAST_ONLY)))
> return -EINVAL;
>
> - if ((gup_flags & FOLL_PIN) && !atomic_read(¤t->mm->has_pinned))
> - atomic_set(¤t->mm->has_pinned, 1);
> + if (gup_flags & FOLL_PIN)
> + mm_set_has_pinned_flag(¤t->mm->flags);
noreply@xxxxxxxxxxxxxx reports:
FAILED linux-next/m5272c3_defconfig/m68k-gcc8 Wed May 12, 19:30
http://kisskb.ellerman.id.au/kisskb/buildresult/14543658/
Commit: Add linux-next specific files for 20210512
ec85c95b0c90a17413901b018e8ade7b9eae7cad
Compiler: m68k-linux-gcc (GCC) 8.1.0 / GNU ld (GNU Binutils) 2.30
mm/gup.c:2698:3: error: implicit declaration of function
'mm_set_has_pinned_flag'; did you mean 'set_tsk_thread_flag'?
[-Werror=implicit-function-declaration]
It's definition is inside the #ifdef CONFIG_MMU section, but the last
user isn't.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
