linux-kernel - Re: [PATCH v5 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

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Message-ID: <87zfyct5y1.fsf@kernel.org>
Date: Fri, 15 Dec 2023 10:31:26 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
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: linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
	Mark Rutland <mark.rutland@....com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rohan McLure <rmclure@...ux.ibm.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	"ndesaulniers@...gle.com" <ndesaulniers@...gle.com>
Subject: Re: [PATCH v5 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.
>

....

> +/*
> + * On shared processor LPARs scheduled on a big core (which has two or more
> + * independent thread groups per core), prefer lower numbered CPUs, so
> + * that workload consolidates to lesser number of cores.
> + */
> +static __ro_after_init DEFINE_STATIC_KEY_FALSE(splpar_asym_pack);


DEFINE_STATIC_KEY_FALSE_RO ?

> +
>  /*
>   * P9 has a slightly odd architecture where pairs of cores share an L2 cache.
>   * This topology makes it *much* cheaper to migrate tasks between adjacent cores
> @@ -1011,9 +1018,20 @@ static int powerpc_smt_flags(void)
>   */

-aneesh