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

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Message-ID: <34ec710f-848a-4f8d-9dac-f9419e9b9c07@amd.com>
Date: Thu, 22 Jan 2026 08:26:29 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Peter Zijlstra <peterz@...radead.org>
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

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.

-- 
Thanks and Regards,
Prateek