linux-kernel - Re: [PATCH] powerpc/smp: Dynamically build powerpc topology

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Message-ID: <20230904221004.GB2568@noisy.programming.kicks-ass.net>
Date:   Tue, 5 Sep 2023 00:10:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        ndesaulniers@...gle.com, Nathan Lynch <nathanl@...ux.ibm.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/smp: Dynamically build powerpc topology

On Wed, Aug 30, 2023 at 05:56:14PM +0530, Srikar Dronamraju wrote:
> Currently there are four powerpc specific sched topologies.  These are
> all statically defined.  However not all these topologies are used by
> all powerpc systems.
> 
> To avoid unnecessary degenerations by the scheduler , masks and flags
> are compared. However if the sched topologies are build dynamically then
> the code is simpler and there are greater chances of avoiding
> degenerations.
> 
> Even x86 builds its sched topologies dynamically and new changes are
> very similar to the way x86 is building its topologies.

You're not stating it explicitly, but you're doing this as a performance
optimization, right? The x86 thing didn't particularly care about
avoiding degenerate topologies -- it's just that the fixed tables method
grew unwieldy due to combinatorics.

And how does this patch relate to the other series touching this?

  powerpc/smp: Shared processor sched optimizations