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

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Message-ID: <20200806085429.GX2674@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Aug 2020 10:54:29 +0200
From:   peterz@...radead.org
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        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

On Thu, Aug 06, 2020 at 03:32:25PM +1000, Michael Ellerman wrote:

> That brings with it a bunch of problems, such as existing software that
> has been developed/configured for Power8 and expects to see SMT8.
> 
> We also allow LPARs to be live migrated from Power8 to Power9 (and back), so
> maintaining the illusion of SMT8 is considered a requirement to make that work.

So how does that work if the kernel booted on P9 and demuxed the SMT8
into 2xSMT4? If you migrate that state onto a P8 with actual SMT8 you're
toast again.

> Yeah I agree the naming is confusing.
> 
> Let's call them "SMT4 cores" and "SMT8 cores"?

Works for me, thanks!

> The problem is we are already lying to userspace, because firmware lies to us.
> 
> ie. the firmware on these systems shows us an SMT8 core, and so current kernels
> show SMT8 to userspace. I don't think we can realistically change that fact now,
> as these systems are already out in the field.
> 
> What this patch tries to do is undo some of the mess, and at least give the
> scheduler the right information.

What a mess... I think it depends on what you do with that P9 to P8
migration case. Does it make sense to have a "p8_compat" boot arg for
the case where you want LPAR migration back onto P8 systems -- in which
case it simply takes the firmware's word as gospel and doesn't untangle
things, because it can actually land on a P8.