Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow path of qspinlock

[Waiman Long]

On 9/22/21 3:25 PM, Davidlohr Bueso 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.

I asked Alex to put in a command line option because we may want to 
disable it on a multi-socket server if we want to.


> > +            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.
I agree that it is now time to look at using the static call for 
slowpath switching.
> > +    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. 

Agreed

Cheers,
Longman