Re: [PATCH v2 4/5] locking/qspinlock: Introduce starvation avoidance into CNA

[Peter Zijlstra]

On Fri, Mar 29, 2019 at 11:20:05AM -0400, Alex Kogan wrote:
> @@ -25,6 +29,18 @@
>  
>  #define MCS_NODE(ptr) ((struct mcs_spinlock *)(ptr))
>  
> +/* Per-CPU pseudo-random number seed */
> +static DEFINE_PER_CPU(u32, seed);
> +
> +/*
> + * Controls the probability for intra-node lock hand-off. It can be
> + * tuned and depend, e.g., on the number of CPUs per node. For now,
> + * choose a value that provides reasonable long-term fairness without
> + * sacrificing performance compared to a version that does not have any
> + * fairness guarantees.
> + */
> +#define INTRA_NODE_HANDOFF_PROB_ARG 0x10000
> +
>  static inline __pure int decode_numa_node(u32 node_and_count)
>  {
>  	int node = (node_and_count >> _Q_NODE_OFFSET) - 1;
> @@ -102,6 +118,35 @@ static struct mcs_spinlock *find_successor(struct mcs_spinlock *me)
>  	return NULL;
>  }
>  
> +/*
> + * xorshift function for generating pseudo-random numbers:
> + * https://en.wikipedia.org/wiki/Xorshift

Cute; so clearly you've read that page, but then you provide us a
variant that isn't actually listed there.

Your naming is also non-standard in that it does not relay the period.
The type seems to suggest 32bit, so the name should then have been:

  xorshift32()

Now, where did you get those parameters from; is this a proper
xorshift32 ?

> + */
> +static inline u32 xor_random(void)
> +{
> +	u32 v;
> +
> +	v = this_cpu_read(seed);
> +	if (v == 0)
> +		get_random_bytes(&v, sizeof(u32));

Given xorshift is a LFSR subset, the above case will only ever happen
_once_ and it seems like bad form to stick it here instead of in a init
function.

Also, does it really matter, can't we simply initialize the variable
with a !0 value and call it a day?

As to that variable, seed is clearly a misnomer, the wiki page you
reference calls it state, which might be a little ambiguous, xs_state
otoh should work just fine.

> +	v ^= v << 6;
> +	v ^= v >> 21;
> +	v ^= v << 7;
> +	this_cpu_write(seed, v);
> +
> +	return v;
> +}

Now, you've read that page and you know there's 'trivial' improvements
on the pure xorshift, why not pick one of those? xorwow seems cheap
enough, or that xorshift128plus() one.

> +
> +/*
> + * Return false with probability 1 / @range.
> + * @range must be a power of 2.
> + */
> +static bool probably(unsigned int range)
> +{
> +	return xor_random() & (range - 1);
> +}

Uhh, you sure that's what it does? The only way for that to return false
is when all @range bits are 0, which happens once (2^32/range)-1 times,
or am I mistaken?

Also, linux/random.h includes next_pseudo_random32(), should we be using
that? Arguably that's more expensive on a number of platforms due to the
multiplication. Also, we actually have xorshift32 already in tree in
lib/test_hash.c.

The advantage of next_psuedo_random32() is that it doesn't have that 0
identify that pure LFSRs suffer from and it has 0 state.  Now at a
glance, the xorwow/xorshift128plus variants don't seem to suffer that 0
identify, so that's good, but they still have fairly large state. It
also seems unfortunate to litter the tree with custom PRNGs. Ted?