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

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Date:   Thu, 19 Oct 2023 15:38:40 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on
 shared processor

Srikar Dronamraju <srikar@...ux.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.

> 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;
> +}
> +
>  /*
>   * 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