Re: [PATCH] futex: Avoid hash-bucket locking for mismatched waits

[Thomas Gleixner]

On Fri, Aug 14 2026 at 18:01, Thomas Gleixner wrote:
> On Mon, Aug 10 2026 at 13:17, Usama Arif wrote:
>> On 07/08/2026 16:42, Thomas Gleixner wrote:
>>> On Wed, Aug 05 2026 at 06:28, Usama Arif wrote:
>>>> On Tue, 4 Aug 2026 17:07:59 +0000 Dmitry Ilvokhin <d@xxxxxxxxxxxx> wrote:
>>>> The above data shows the significance of the patch.
>>>> It provides a very meaningful improvement (22.4% of time spent in futex_q_lock()
>>>> will be significantly optimized and will also deliver second-order effects)
>>>> and has no measurable impact on latency in the matching path.
>>>> IMHO, this patch is a free lunch.
>>> 
>>> Not really free. The user space access is not exactly cheap either
>>> because CLAC/STAC are memory fencing to meet the SMAP guarantees.
>>
>> My understanding from 86e6b1547b3d is that STAC/CLAC “end up serializing
>> execution on older Zen,” while Zen 5’s AC renaming “improves performance
>> of STAC/CLAC a lot a lot.”  Architecturally, they only change the AC bit.
>> They are not memory-ordering instructions like LFENCE.
>
> It's not a memory ordering instruction, but it has to guarantee that the
> AC change is effective when the subsequent permission check
> happens. That's true for both STAC and CLAC.
>
> So it _cannot_ be free by definition and the penalty depends on the
> micro architecture.
>
>> I am currently testing on Zen5 which could be why I didn't see any
>> wall-time regression in futex_wait_timeout.c from [1].
>
> It's not relevant whether your ZEN5 works fine or not. We are not
> optimizing for a particular machine.
>
> A trivial futex bouncing test case with two threads degrades on a ZEN3
> by ~20% and when looking at it with perf top clearly the extra user
> access stands out very prominently.
>
> The below variant does not expose that behavior and actually improves
> the same test case by ~5% on that machine.

Bah. Included the broken version. Fixed one is below.

Thanks,

        tglx
---
 kernel/futex/waitwake.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -857,7 +857,21 @@ int futex_wait_setup(u32 __user *uaddr,
 		CLASS(hbr, hbr)(&q->key);
 		auto hb = hbr.hb;
 
-		futex_q_lock(q, hb);
+		futex_hb_waiters_inc(hb);
+		q->lock_ptr = &hb->lock;
+
+		if (!spin_trylock(&hb->lock)) {
+			ret = get_user_inline(uval, uaddr);
+			if (ret) {
+				futex_hb_waiters_dec(hb);
+				return ret;
+			}
+			if (uval != val) {
+				futex_hb_waiters_dec(hb);
+				return -EWOULDBLOCK;
+			}
+			spin_lock(&hb->lock);
+		}
 
 		ret = futex_get_value_locked(&uval, uaddr);