linux-kernel - Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt

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Message-ID: <20200806140955.GC31068@linux.vnet.ibm.com>
Date:   Thu, 6 Aug 2020 19:39:55 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     peterz@...radead.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>
Subject: Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask

* peterz@...radead.org <peterz@...radead.org> [2020-08-06 15:15:47]:

> > But my understanding is most LPARs don't get migrated back and forth,
> > they'll start life on a P8 and only get migrated to a P9 once when the
> > customer gets a P9. They might then run for a long time (months to
> > years) on the P9 in P8 compat mode, not because they ever want to
> > migrate back to a real P8, but because the software in the LPAR is still
> > expecting to be on a P8.
> > 
> > I'm not a real expert on all the Enterprisey stuff though, so someone
> > else might be able to give us a better picture.
> > 
> > But the point of mentioning the migration stuff was mainly just to
> > explain why we feel we need to present SMT8 to userspace even on P9.
> 
> OK, fair enough. The patch wasn't particularly onerous, I was just
> wondering why etc..
> 
> The case of starting on a P8 and being migrated to a P9 makes sense to
> me; in that case you'd like to rebuild your sched domains, but can't go
> about changing user visible topolofy information.
> 
> I suppose:
> 
> Acked-by; Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> An updated Changelog that recaps some of this discussion might also be
> nice.

Okay, will surely do the needful.

-- 
Thanks and Regards
Srikar Dronamraju