Re: [PATCH v9 1/8] mm: Add per-cpu logic to page shuffling

Alexander Duyck
2019-09-09T15:12:15Z

On Mon, 2019-09-09 at 12:07 +0300, Kirill A. Shutemov wrote:
> On Sat, Sep 07, 2019 at 10:25:12AM -0700, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >
> > Change the logic used to generate randomness in the suffle path so that we
>
> Typo.
>
> > can avoid cache line bouncing. The previous logic was sharing the offset
> > and entropy word between all CPUs. As such this can result in cache line
> > bouncing and will ultimately hurt performance when enabled.
> >
> > To resolve this I have moved to a per-cpu logic for maintaining a unsigned
> > long containing some amount of bits, and an offset value for which bit we
> > can use for entropy with each call.
> >
> > Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> > mm/shuffle.c | 33 +++++++++++++++++++++++----------
> > 1 file changed, 23 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/shuffle.c b/mm/shuffle.c
> > index 3ce12481b1dc..9ba542ecf335 100644
> > --- a/mm/shuffle.c
> > +++ b/mm/shuffle.c
> > @@ -183,25 +183,38 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
> > shuffle_zone(z);
> > }
> >
> > +struct batched_bit_entropy {
> > + unsigned long entropy_bool;
> > + int position;
> > +};
> > +
> > +static DEFINE_PER_CPU(struct batched_bit_entropy, batched_entropy_bool);
> > +
> > void add_to_free_area_random(struct page *page, struct free_area *area,
> > int migratetype)
> > {
> > - static u64 rand;
> > - static u8 rand_bits;
> > + struct batched_bit_entropy *batch;
> > + unsigned long entropy;
> > + int position;
> >
> > /*
> > - * The lack of locking is deliberate. If 2 threads race to
> > - * update the rand state it just adds to the entropy.
> > + * We shouldn't need to disable IRQs as the only caller is
> > + * __free_one_page and it should only be called with the zone lock
> > + * held and either from IRQ context or with local IRQs disabled.
> > */
> > - if (rand_bits == 0) {
> > - rand_bits = 64;
> > - rand = get_random_u64();
> > + batch = raw_cpu_ptr(&batched_entropy_bool);
> > + position = batch->position;
> > +
> > + if (--position < 0) {
> > + batch->entropy_bool = get_random_long();
> > + position = BITS_PER_LONG - 1;
> > }
> >
> > - if (rand & 1)
> > + batch->position = position;
> > + entropy = batch->entropy_bool;
> > +
> > + if (1ul & (entropy >> position))
>
> Maybe something like this would be more readble:
>
> if (entropy & BIT(position))
>
> > add_to_free_area(page, area, migratetype);
> > else
> > add_to_free_area_tail(page, area, migratetype);
> > - rand_bits--;
> > - rand >>= 1;
> > }
> >
> >

Thanks for the review. I will update these two items for v10.

- Alex