Messages in this thread | | | Date | Wed, 4 Jan 2023 21:19:44 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] locking/qspinlock: Optimize pending state waiting for unlock |
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* Guo Ren <guoren@kernel.org> wrote:
> > >> The situation is the SMT scenarios in the same core. Not an entering > > >> low-power state situation. Of course, the granularity between cores is > > >> "cacheline", but the granularity between SMT hw threads of the same > > >> core could be "byte" which internal LSU handles. For example, when a > > >> hw-thread yields the resources of the core to other hw-threads, this > > >> patch could help the hw-thread stay in the sleep state and prevent it > > >> from being woken up by other hw-threads xchg_tail. > > >> > > >> Finally, from the software semantic view, does the patch make it more > > >> accurate? (We don't care about the tail here.) > > > > > > Thanks for the clarification. > > > > > > I am not arguing for the simplification part. I just want to clarify > > > my limited understanding of how the CPU hardware are actually dealing > > > with these conditions. > > > > > > With that, I am fine with this patch. It would be nice if you can > > > elaborate a bit more in your commit log. > > > > > > Acked-by: Waiman Long <longman@redhat.com> > > > > > BTW, have you actually observe any performance improvement with this patch? > Not yet. I'm researching how the hardware could satisfy qspinlock > better. Here are three points I concluded: > 1. Atomic forward progress guarantee: Prevent unnecessary LL/SC > retry, which may cause expensive bus transactions when crossing the > NUMA nodes. > 2. Sub-word atomic primitive: Enable freedom from interference > between locked, pending, and tail. > 3. Load-cond primitive: Prevent processor from wasting loop > operations for detection.
As to this patch, please send a -v2 version of this patch that has this discussion & explanation included in the changelog, as requested by Waiman.
Thanks,
Ingo
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