Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression

Peter Zijlstra
2020-03-09T08:52:32Z

On Sun, Mar 08, 2020 at 07:07:17PM +0100, Thomas Gleixner wrote:

> Right you are. The pointer needs to be the starting point as it moved
> ahead of word, which means it starts at word and hashes word and
> offset and an extra u32 beyond the end of the key.
>
> Thanks,
>
> tglx
> ----
> diff --git a/kernel/futex.c b/kernel/futex.c
> index e14f7cd45dbd..9f3251349f65 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -385,8 +385,8 @@ static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
> */
> static struct futex_hash_bucket *hash_futex(union futex_key *key)
> {
> - u32 hash = jhash2((u32*)&key->both.word,
> - (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
> + u32 hash = jhash2((u32*)&key->both.ptr,
> + (sizeof(key->both.ptr) + sizeof(key->both.word)) / 4,
> key->both.offset);
> return &futex_queues[hash & (futex_hashsize - 1)];
> }

Groan... I've gotta ask, why isn't that written like:

u32 hash = jhash2((u32 *)key,
offsetof(typeof(*key), offset) / 4
key->both.offset);

Or better yet:

u32 hash = jhash((u32 *)key, sizeof(*key) / 4, 0);