linux-kernel - Re: [futex] 8019ad13ef: will-it-scale.per_process

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Date:   Mon, 09 Mar 2020 10:34:48 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        kernel test robot <rong.a.chen@...el.com>,
        Jann Horn <jannh@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, x86-ml <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        lkp@...ts.01.org
Subject: Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression

Peter Zijlstra <peterz@...radead.org> writes:
> On Sun, Mar 08, 2020 at 07:07:17PM +0100, Thomas Gleixner wrote:
>>  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);

Histerical raisins :)