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 <[email protected]> > > > > 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 <[email protected]> > > Signed-off-by: Alexander Duyck <[email protected]> > > --- > > 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
