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

From: Alexander Duyck <[email-masked]>
Date: Mon, 09 Sep 2019 08:12:12 -0700

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

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