linux-kernel - Re: [PATCH] futex: Use a hashmask instead of hashsize.

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Message-ID: <44f36f13-4ce6-47ce-9423-9e4cceda7443@redhat.com>
Date: Wed, 26 Feb 2025 09:37:26 -0500
From: Waiman Long <llong@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, André Almeida
 <andrealmeid@...lia.com>, Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
 Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] futex: Use a hashmask instead of hashsize.

On 2/26/25 4:10 AM, Sebastian Andrzej Siewior wrote:
> The global hash uses futex_hashsize to save the amount of the hash
> buckets that have been allocated during system boot. On each
> futex_hash() invocation this number is substracted by one to get the
> mask. This can be optimized by saving directly the mask avoiding the
> substraction on each futex_hash() invocation.
>
> Rename futex_hashsize to futex_hashmask and save the mask of the
> allocated hash map.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>
> Split from the "local hash" series.
>
>   kernel/futex/core.c | 21 +++++++++++----------
>   1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 3db8567f5a44e..cca15859a50be 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -50,10 +50,10 @@
>    */
>   static struct {
>   	struct futex_hash_bucket *queues;
> -	unsigned long            hashsize;
> +	unsigned long            hashmask;
>   } __futex_data __read_mostly __aligned(2*sizeof(long));
>   #define futex_queues   (__futex_data.queues)
> -#define futex_hashsize (__futex_data.hashsize)
> +#define futex_hashmask (__futex_data.hashmask)
>   
>   
>   /*
> @@ -119,7 +119,7 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
>   	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
>   			  key->both.offset);
>   
> -	return &futex_queues[hash & (futex_hashsize - 1)];
> +	return &futex_queues[hash & futex_hashmask];
>   }
>   
>   
> @@ -1127,27 +1127,28 @@ void futex_exit_release(struct task_struct *tsk)
>   
>   static int __init futex_init(void)
>   {
> +	unsigned long hashsize, i;
>   	unsigned int futex_shift;
> -	unsigned long i;
>   
>   #ifdef CONFIG_BASE_SMALL
> -	futex_hashsize = 16;
> +	hashsize = 16;
>   #else
> -	futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
> +	hashsize = roundup_pow_of_two(256 * num_possible_cpus());
>   #endif
>   
>   	futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
> -					       futex_hashsize, 0, 0,
> +					       hashsize, 0, 0,
>   					       &futex_shift, NULL,
> -					       futex_hashsize, futex_hashsize);
> -	futex_hashsize = 1UL << futex_shift;
> +					       hashsize, hashsize);
> +	hashsize = 1UL << futex_shift;
>   
> -	for (i = 0; i < futex_hashsize; i++) {
> +	for (i = 0; i < hashsize; i++) {
>   		atomic_set(&futex_queues[i].waiters, 0);
>   		plist_head_init(&futex_queues[i].chain);
>   		spin_lock_init(&futex_queues[i].lock);
>   	}
>   
> +	futex_hashmask = hashsize - 1;
>   	return 0;
>   }
>   core_initcall(futex_init);

LGTM

Reviewed-by: Waiman Long <longman@...hat.com>