io_uring/futex: return a wait's data to the cache when it fails to arm

io_futex_wait() takes its io_futex_data from ctx->futex_cache, and a
completed wait gives it back in io_futex_complete().  A wait that fails
to arm kfree()s it instead.

The usual failure is -EAGAIN from futex_wait_setup(): the futex word no
longer holds the expected value.  That is routine for a waiter that
re-arms with the value it last read, since the word changes whenever
there is something to wake for.  Each such failure takes an object out
of the cache for good, so the cache drains and from then on waits
allocate.

The cache exists so that arming a wait doesn't have to go to the slab
allocator, and that matters most to the waiters that re-arm the most:
event loops, often real-time ones.  A kmalloc() or kfree() that misses
the per-CPU slab takes the node's list_lock, shared with every other
user of that kmalloc size on the system, and may go on to the page
allocator.

On PREEMPT_RT list_lock is a sleeping lock.  A real-time task that
finds it held waits for the holder, which may be any task on any CPU,
to get to run and release it.  Priority inheritance boosts the holder
only up to the waiter's priority, so the waiter still sits behind
whatever outranks that on the holder's CPU, such as threaded interrupt
handlers.  Re-arming a futex wait should not depend on any of that.

Put the data back in the cache on that path too, while the submit lock
still protects it.

In an event loop re-arming one IORING_OP_FUTEX_WAIT per message under
load, 0.39 arms per message failed with -EAGAIN, and io_futex_wait() did
0.39 kmalloc()s and kfree()s per message.  With this change, 100,000
waits that all fail with -EAGAIN do one kmalloc() in total.

Fixes: 194bb58c6090 ("io_uring: add support for futex wake and wait")
Assisted-by: LLM
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
Link: https://patch.msgid.link/20261019231243.802569-1-austin.linux@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions