Re: [PATCH 0/4] io_uring: improve alloc_cache branch prediction

[Jens Axboe]

On 11/3/26 2:10 PM, Caleb Sander Mateos wrote:
> A call to io_uring_alloc_async_data() that provides a struct io_alloc_cache
> currently has 2 unnecessary NULL checks in the fast path:
> - Of the struct io_alloc_cache *, even though it points to a struct io_ring_ctx
>   field
> - Of the void * entry popped from the cache, even though all cache entries are
>   non-NULL
> The compiler also incorrectly assumes the success path in io_alloc_cache_get()
> is the unlikely one because it's written as an early return.
> 
> Avoid the first NULL check by splitting io_uring_alloc_async_data()'s cache
> logic to a separate function io_uring_alloc_cached_async_data() that uses the
> cache unconditionally. Avoid the second by returning a separate success bool
> from io_cache_alloc() instead of checking the NULL-ness of the returned entry.
> Also mark io_alloc_cache_get() and io_alloc_cache_put()'s success paths as
> likely so the compiler can avoid jumps in the fast path.
> 
> The final commit is a small code simplification in io_futex_wait() noticed while
> touching the neighboring io_cache_alloc() call.

I hand-applied this, as it needed changes for the post-7.3 trees.
Patch 3 is somewhat marginal I'd say in terms of rewards for the
churn, but churn is cheap these days...

Anyway, check the end result, please.

-- 
Jens Axboe