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 <[email protected]>
>>>> 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);
