linux-kernel - Re: [PATCH v3 7/7] sched/topology: Use prebuilt SD flag degeneration mask

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Message-ID: <20200713125543.GJ10769@hirez.programming.kicks-ass.net>
Date:   Mon, 13 Jul 2020 14:55:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        morten.rasmussen@....com
Subject: Re: [PATCH v3 7/7] sched/topology: Use prebuilt SD flag degeneration
 mask

On Wed, Jul 01, 2020 at 08:06:55PM +0100, Valentin Schneider wrote:
> Leverage SD_DEGENERATE_GROUPS_MASK in sd_degenerate() and
> sd_degenerate_parent().
> 
> Note that this changes sd_degenerate() somewhat: I'm using the negation of
> SD_DEGENERATE_GROUPS_MASK as the mask of flags not requiring groups, which
> is equivalent to:
> 
> SD_WAKE_AFFINE | SD_SERIALIZE | SD_NUMA
> 
> whereas the current mask for that is simply
> 
> SD_WAKE_AFFINE
> 
> I played with a few toy NUMA topologies on QEMU and couldn't cause a
> different degeneration than what mainline does currently. If that is deemed
> too risky, we can go back to using SD_WAKE_AFFINE explicitly.

Arguably SD_SERIALIZE needs groups, note how we're only having that
effective for machines with at least 2 nodes. It's a bit shit how we end
up there, but IIRC that's what it ends up as.

SD_NUMA is descriptive, and not marking a group as degenerates because
it has SD_NUMA seems a bit silly. But then, it would be the top domain
and would survive anyway?