linux-kernel - Re: [RFC PATCH 3/3] futex: Use the task local hashmap.

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Message-ID: <20241028102253.GN9767@noisy.programming.kicks-ass.net>
Date: Mon, 28 Oct 2024 11:22:53 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...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>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [RFC PATCH 3/3] futex: Use the task local hashmap.

On Sun, Oct 27, 2024 at 12:34:52AM +0200, Sebastian Andrzej Siewior wrote:
> Use the hashlocal hashmap if provided.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/futex/core.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 7c97fc96f84a3..ee5a6902154c5 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -119,13 +119,18 @@ late_initcall(fail_futex_debugfs);
>   * @key:	Pointer to the futex key for which the hash is calculated
>   *
>   * We hash on the keys returned from get_futex_key (see below) and return the
> - * corresponding hash bucket in the global hash.
> + * corresponding hash bucket in the global hash. If the FUTEX is private and
> + * a local hash table is privated then this one is used.
>   */
>  struct futex_hash_bucket *futex_hash(union futex_key *key)
>  {
> +	struct futex_hash_table *fht;
>  	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
>  			  key->both.offset);
>  
> +	fht = current->futex_hash_table;
> +	if (fht && (key->both.offset & (FUT_OFF_INODE | FUT_OFF_MMSHARED)) == 0)
> +		return &fht->queues[hash & (fht->slots - 1)];

Perhaps add a helper like:

static inline bool futex_key_is_private(struct futex_key *key)
{
	/*
	 * Relies on get_futex_key() to set either bit for shared
	 * futexes -- see comment with union futex_key.
	 */
	return !(key->both.offset & (FUT_OFF_INODE | FUT_OFF_MMSHARED));
}