linux-kernel - Re: [PATCH v3 0/8] sched/topology: Optimize sd->shared allocation

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Message-ID: <20260123095455.GI171111@noisy.programming.kicks-ass.net>
Date: Fri, 23 Jan 2026 10:54:55 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH v3 0/8] sched/topology: Optimize sd->shared allocation

On Thu, Jan 22, 2026 at 08:26:29AM +0530, K Prateek Nayak wrote:
> Hello Peter,
> 
> On 1/21/2026 9:46 PM, Peter Zijlstra wrote:
> > On Tue, Jan 20, 2026 at 11:32:38AM +0000, K Prateek Nayak wrote:
> > 
> >> "sd->shared" is only allocated for the topmost SD_SHARE_LLC domain and
> >> the topology layer uses the sched domain degeneration path to pass the
> >> reference to the final "sd_llc" domain. 
> > 
> > I'm fairly sure we've had patches that introduced it for other levels at
> > various times, but clearly none of those ever made it.
> > 
> > Anyway, a quick peek seems to suggest it is still easy to extend.
> > 
> > 
> >>  include/linux/sched/topology.h |   1 -
> >>  kernel/sched/fair.c            |  62 +++++++-----------
> >>  kernel/sched/sched.h           |   2 +-
> >>  kernel/sched/topology.c        | 111 ++++++++++++++++++++++-----------
> >>  4 files changed, 101 insertions(+), 75 deletions(-)
> > 
> > Is this really worth the extra lines though?
> 
> The larger plan was to move the "nohz.idle_cpus" tracking into the
> sched_domain_shared instance which will bloat these allocations.
> 
> Instead of (#CPUs x #topology_levels) surplus, most of which will get
> reclaimed at the end anyways, we'll only have #CPUs worth of
> allocations now.

Fair enough I suppose. Be sure to call this out as the primary reason
for doing this.