Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on shared processor

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 19, 2023 at 03:38:40PM +1100, Michael Ellerman wrote:
> Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:
> > If there are shared processor LPARs, underlying Hypervisor can have more
> > virtual cores to handle than actual physical cores.
> >
> > Starting with Power 9, a core has 2 nearly independent thread groups.
> 
> You need to be clearer here that you're talking about "big cores", not
> SMT4 cores as seen on bare metal systems.

What is a 'big core' ? I'm thinking big.LITTLE, but I didn't think Power
went that route (yet?).. help?

> > On a shared processors LPARs, it helps to pack threads to lesser number
> > of cores so that the overall system performance and utilization
> > improves. PowerVM schedules at a core level. Hence packing to fewer
> > cores helps.
> >
> > For example: Lets says there are two 8-core Shared LPARs that are
> > actually sharing a 8 Core shared physical pool, each running 8 threads
> > each. Then Consolidating 8 threads to 4 cores on each LPAR would help
> > them to perform better. This is because each of the LPAR will get
> > 100% time to run applications and there will no switching required by
> > the Hypervisor.
> >
> > To achieve this, enable SD_ASYM_PACKING flag at CACHE, MC and DIE level.
> 
> .. when the system is running in shared processor mode and has big cores.
> 
> cheers
> 
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 37c41297c9ce..498c2d51fc20 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -1009,9 +1009,20 @@ static int powerpc_smt_flags(void)
> >   */
> >  static int powerpc_shared_cache_flags(void)
> >  {
> > +	if (static_branch_unlikely(&powerpc_asym_packing))
> > +		return SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
> > +
> >  	return SD_SHARE_PKG_RESOURCES;
> >  }
> >  
> > +static int powerpc_shared_proc_flags(void)
> > +{
> > +	if (static_branch_unlikely(&powerpc_asym_packing))
> > +		return SD_ASYM_PACKING;
> > +
> > +	return 0;
> > +}

Can you leave the future reader a clue in the form of a comment around
here perhaps? Explaining *why* things are as they are etc..

> > +
> >  /*
> >   * We can't just pass cpu_l2_cache_mask() directly because
> >   * returns a non-const pointer and the compiler barfs on that.
> > @@ -1048,8 +1059,8 @@ static struct sched_domain_topology_level powerpc_topology[] = {
> >  	{ cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
> >  #endif
> >  	{ shared_cache_mask, powerpc_shared_cache_flags, SD_INIT_NAME(CACHE) },
> > -	{ cpu_mc_mask, SD_INIT_NAME(MC) },
> > -	{ cpu_cpu_mask, SD_INIT_NAME(DIE) },
> > +	{ cpu_mc_mask, powerpc_shared_proc_flags, SD_INIT_NAME(MC) },
> > +	{ cpu_cpu_mask, powerpc_shared_proc_flags, SD_INIT_NAME(DIE) },
> >  	{ NULL, },
> >  };
> >  
> > @@ -1687,6 +1698,8 @@ static void __init fixup_topology(void)
> >  	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
> >  		pr_info_once("Enabling Asymmetric SMT scheduling\n");
> >  		static_branch_enable(&powerpc_asym_packing);
> > +	} else if (is_shared_processor() && has_big_cores) {
> > +		static_branch_enable(&powerpc_asym_packing);
> >  	}
> >  
> >  #ifdef CONFIG_SCHED_SMT
> > -- 
> > 2.31.1