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

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	kernel test robot <rong.a.chen@intel.com>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	x86-ml <x86@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Borislav Petkov <bp@suse.de>,
	lkp@lists.01.org
Subject: Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression
Date: Mon, 9 Mar 2020 09:51:23 +0100	[thread overview]
Message-ID: <20200309085123.GE12561@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87h7yy90ve.fsf@nanos.tec.linutronix.de>

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);