Re: [PATCH v8 03/15] futex: Add basic infrastructure for local task local hash.

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 03, 2025 at 02:59:23PM +0100, Sebastian Andrzej Siewior wrote:
> The futex hashmap is system wide and shared by random tasks. Each slot
> is hashed based on its address and VMA. Due to randomized VMAs (and
> memory allocations) the same logical lock (pointer) can end up in a
> different hash bucket on each invocation of the application. This in
> turn means that different applications may share a hash bucket on the
> first invocation but not on the second an it is not always clear which
> applications will be involved. This can result in high latency's to
> acquire the futex_hash_bucket::lock especially if the lock owner is
> limited to a CPU and not be effectively PI boosted.
> 
> Introduce a task local hash map. The hashmap can be allocated via
> 	prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS, 0)
> 
> The `0' argument allocates a default number of 16 slots, a higher number
> can be specified if desired. The current upper limit is 131072.

Hmm, I would expect 0 to disable the local thing.

Now, I realize this is somewhat tricky, since there might be futexes
inside it. But mapping 0 to some default value seems.. well, odd.