linux-kernel - Re: [PATCH v4 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

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Date:   Wed, 15 Nov 2023 12:05:22 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Mark Rutland <mark.rutland@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Rohan McLure <rmclure@...ux.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v4 1/5] 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 big core (aka SMT8 core) has 2 nearly
> independent thread groups. 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 big 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.
>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> ---
> Changelog:
> v3 -> v4:
> - Dont use splpar_asym_pack with SMT
> - Conflict resolution due to rebase
> 	(DIE changed to PKG)
> v2 -> v3:
> - Handle comments from Michael Ellerman.
> - Rework using existing cpu_has_features static key
> v1->v2: Using Jump label instead of a variable.
>
>  arch/powerpc/kernel/smp.c | 37 +++++++++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index ab691c89d787..69a3262024f1 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -993,16 +993,20 @@ static bool shared_caches;
>  /* cpumask of CPUs with asymmetric SMT dependency */
>  static int powerpc_smt_flags(void)
>  {
> -	int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
> +	if (!cpu_has_feature(CPU_FTR_ASYM_SMT))
> +		return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
>  
> -	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
> -		printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
> -		flags |= SD_ASYM_PACKING;
> -	}
> -	return flags;
> +	return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
>  }
>  #endif
>

Only relevant change there is dropping printk_once(). Rest of the
changes are not needed?

-aneesh