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SubjectRe: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow path of qspinlock
On Thu, Sep 23, 2021 at 3:26 AM Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> On Fri, 14 May 2021, Alex Kogan wrote:
>
> >diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> >index a816935d23d4..94d35507560c 100644
> >--- a/Documentation/admin-guide/kernel-parameters.txt
> >+++ b/Documentation/admin-guide/kernel-parameters.txt
> >@@ -3515,6 +3515,16 @@
> > NUMA balancing.
> > Allowed values are enable and disable
> >
> >+ numa_spinlock= [NUMA, PV_OPS] Select the NUMA-aware variant
> >+ of spinlock. The options are:
> >+ auto - Enable this variant if running on a multi-node
> >+ machine in native environment.
> >+ on - Unconditionally enable this variant.
>
> Is there any reason why the user would explicitly pass the on option
> when the auto thing already does the multi-node check? Perhaps strange
> numa topologies? Otherwise I would say it's not needed and the fewer
> options we give the user for low level locking the better.
>
> >+ off - Unconditionally disable this variant.
> >+
> >+ Not specifying this option is equivalent to
> >+ numa_spinlock=auto.
> >+
> > numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
> > 'node', 'default' can be specified
> > This can be set from sysctl after boot.
> >diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >index 0045e1b44190..819c3dad8afc 100644
> >--- a/arch/x86/Kconfig
> >+++ b/arch/x86/Kconfig
> >@@ -1564,6 +1564,26 @@ config NUMA
> >
> > Otherwise, you should say N.
> >
> >+config NUMA_AWARE_SPINLOCKS
> >+ bool "Numa-aware spinlocks"
> >+ depends on NUMA
> >+ depends on QUEUED_SPINLOCKS
> >+ depends on 64BIT
> >+ # For now, we depend on PARAVIRT_SPINLOCKS to make the patching work.
> >+ # This is awkward, but hopefully would be resolved once static_call()
> >+ # is available.
> >+ depends on PARAVIRT_SPINLOCKS
>
> We now have static_call() - see 9183c3f9ed7.
>
>
> >+ default y
> >+ help
> >+ Introduce NUMA (Non Uniform Memory Access) awareness into
> >+ the slow path of spinlocks.
> >+
> >+ In this variant of qspinlock, the kernel will try to keep the lock
> >+ on the same node, thus reducing the number of remote cache misses,
> >+ while trading some of the short term fairness for better performance.
> >+
> >+ Say N if you want absolute first come first serve fairness.
>
> This would also need a depends on !PREEMPT_RT, no? Raw spinlocks really want
> the determinism.
I hope we shouldn't force disable it in the Kconfig. Could we put this
idea in numa_spinlock=auto?

>
> Thanks,
> Davidlohr



--
Best Regards
Guo Ren

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