Re: [PATCH v2 11/14] futex: Implement FUTEX2_NUMA

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 07, 2023 at 11:11:04PM +0200, Thomas Gleixner wrote:
> On Mon, Aug 07 2023 at 14:18, Peter Zijlstra wrote:
> >  /**
> >   * futex_hash - Return the hash bucket in the global hash
> >   * @key:	Pointer to the futex key for which the hash is calculated
> > @@ -114,10 +137,29 @@ late_initcall(fail_futex_debugfs);
> >   */
> >  struct futex_hash_bucket *futex_hash(union futex_key *key)
> >  {
> > -	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
> > +	u32 hash = jhash2((u32 *)key,
> > +			  offsetof(typeof(*key), both.offset) / sizeof(u32),
> >  			  key->both.offset);
> > +	int node = key->both.node;
> > +
> > +	if (node == -1) {
> 
> NUMA_NO_NODE please all over the place.

Ah, so our (futex2) ABI states this needs to be -1, but in theory
someone could come along and change the kernel internal NUMA_NO_NODE to
something else.

That is, I explicitly chose not to use it. I can of course, because as
of now these values do match.

> > +		/*
> > +		 * In case of !FLAGS_NUMA, use some unused hash bits to pick a
> > +		 * node -- this ensures regular futexes are interleaved across
> > +		 * the nodes and avoids having to allocate multiple
> > +		 * hash-tables.
> > +		 *
> > +		 * NOTE: this isn't perfectly uniform, but it is fast and
> > +		 * handles sparse node masks.
> > +		 */
> > +		node = (hash >> futex_hashshift) % nr_node_ids;
> > +		if (!node_possible(node)) {
> > +			node = find_next_bit_wrap(node_possible_map.bits,
> > +						  nr_node_ids, node);
> > +		}
> 
> Smart.

Thanks :-)

> >  
> > +static inline unsigned int futex_size(unsigned int flags)
> > +{
> > +	return 1 << (flags & FLAGS_SIZE_MASK);
> > +}
> > +
> >  static inline bool futex_flags_valid(unsigned int flags)
> 
> If you reorder these two functions in the patch which introduces them,
> this diff gets readable :)

Durr, I knew I was forgetting something, will do.