[Deepin-Kernel-SIG] [linux 6.18.y] [Upstream] sched_ext: idle: Prioritize idle SMT sibling by Avenger-285714 · Pull Request #1743 · deepin-community/kernel

Avenger-285714 · 2026-05-18T09:24:09Z

[Upstream commit 2197cecdb02c57b08340059452540fcf101fa30d]

In the default built-in idle CPU selection policy, when @prev_cpu is busy and no fully idle core is available, try to place the task on its SMT sibling if that sibling is idle, before searching any other idle CPU in the same LLC.

Migration to the sibling is cheap and keeps the task on the same core, preserving L1 cache and reducing wakeup latency.

On large SMT systems this appears to consistently boost throughput by roughly 2-3% on CPU-bound workloads (running a number of tasks equal to the number of SMT cores).

Cc: Cheng-Yang Chou yphbchou0911@gmail.com

Summary by Sourcery

Enhancements:

Update the default idle CPU selection policy to first select an idle SMT sibling of the previous CPU when the previous CPU is busy and no fully idle core is available.

[Upstream commit 2197cecdb02c57b08340059452540fcf101fa30d] In the default built-in idle CPU selection policy, when @prev_cpu is busy and no fully idle core is available, try to place the task on its SMT sibling if that sibling is idle, before searching any other idle CPU in the same LLC. Migration to the sibling is cheap and keeps the task on the same core, preserving L1 cache and reducing wakeup latency. On large SMT systems this appears to consistently boost throughput by roughly 2-3% on CPU-bound workloads (running a number of tasks equal to the number of SMT cores). Cc: Cheng-Yang Chou <yphbchou0911@gmail.com> Signed-off-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: WangYuli <wangyl5933@chinaunicom.cn>

sourcery-ai · 2026-05-18T09:24:16Z

Reviewer's Guide

This PR updates the default sched_ext idle CPU selection policy to prefer scheduling on an idle SMT sibling of the previous CPU before searching other idle CPUs in the same LLC, and updates the documented selection steps accordingly.

File-Level Changes

Change	Details	Files
Prefer an idle SMT sibling of the previous CPU in the default sched_ext idle CPU selection path before scanning the LLC for idle CPUs.	Updated the documented step ordering in the scx_select_cpu_dfl policy comment block to insert a new SMT-sibling preference step and renumber subsequent steps. Added a runtime check using sched_smt_active() and for_each_cpu_and() over cpu_smt_mask(prev_cpu) intersected with the allowed mask to scan prev_cpu's SMT siblings. Within that loop, skipped prev_cpu itself and used scx_idle_test_and_clear_cpu() on each sibling, returning early when an idle sibling is found before falling back to LLC-based idle CPU search.	`kernel/sched/ext_idle.c`

Tips and commands

Interacting with Sourcery

Trigger a new review: Comment @sourcery-ai review on the pull request.
Continue discussions: Reply directly to Sourcery's review comments.
Generate a GitHub issue from a review comment: Ask Sourcery to create an
issue from a review comment by replying to it. You can also reply to a
review comment with @sourcery-ai issue to create an issue from it.
Generate a pull request title: Write @sourcery-ai anywhere in the pull
request title to generate a title at any time. You can also comment
@sourcery-ai title on the pull request to (re-)generate the title at any time.
Generate a pull request summary: Write @sourcery-ai summary anywhere in
the pull request body to generate a PR summary at any time exactly where you
want it. You can also comment @sourcery-ai summary on the pull request to
(re-)generate the summary at any time.
Generate reviewer's guide: Comment @sourcery-ai guide on the pull
request to (re-)generate the reviewer's guide at any time.
Resolve all Sourcery comments: Comment @sourcery-ai resolve on the
pull request to resolve all Sourcery comments. Useful if you've already
addressed all the comments and don't want to see them anymore.
Dismiss all Sourcery reviews: Comment @sourcery-ai dismiss on the pull
request to dismiss all existing Sourcery reviews. Especially useful if you
want to start fresh with a new review - don't forget to comment
@sourcery-ai review to trigger a new review!

Customizing Your Experience

Access your dashboard to:

Enable or disable review features such as the Sourcery-generated pull request
summary, the reviewer's guide, and others.
Change the review language.
Add, remove or edit custom review instructions.
Adjust other review settings.

Getting Help

Contact our support team for questions or feedback.
Visit our documentation for detailed guides and information.
Keep in touch with the Sourcery team by following us on X/Twitter, LinkedIn or GitHub.

deepin-ci-robot · 2026-05-18T09:24:18Z

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from avenger-285714. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

deepin/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

sourcery-ai

Hey - I've reviewed your changes and they look great!

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

Copilot

Pull request overview

Updates the default sched_ext built-in idle CPU selection policy to prefer waking/migrating onto an idle SMT sibling of prev_cpu before searching wider domains, aiming to keep work on the same physical core for better cache locality and lower wakeup latency.

Changes:

Document the new selection step to prioritize prev_cpu’s idle SMT sibling when no fully-idle core is available.
Add an SMT-sibling idle-CPU scan in scx_select_cpu_dfl() before the LLC/NUMA idle searches.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

+	/*
+	 * Use @prev_cpu's sibling if it's idle.
+	 */
+	if (sched_smt_active()) {
+		for_each_cpu_and(cpu, cpu_smt_mask(prev_cpu), allowed) {
+			if (cpu == prev_cpu)
+				continue;
+			if (scx_idle_test_and_clear_cpu(cpu))
+				goto out_unlock;
+		}
+	}


opsiff · 2026-05-18T10:25:20Z

commit 63f500c32a37d490ec623a3130e488cdb9bd6cf7
Author: Andrea Righi arighi@nvidia.com
Date: Sun Mar 22 07:51:46 2026 +0100

sched_ext: Guard cpu_smt_mask() with CONFIG_SCHED_SMT

Wrap cpu_smt_mask() usage with CONFIG_SCHED_SMT to avoid build failures
on kernels built without SMT support.

Fixes: 2197cecdb02c ("sched_ext: idle: Prioritize idle SMT sibling")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603221422.XIueJOE9-lkp@intel.com/
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

[Upstream commit 63f500c32a37d490ec623a3130e488cdb9bd6cf7] Wrap cpu_smt_mask() usage with CONFIG_SCHED_SMT to avoid build failures on kernels built without SMT support. Fixes: 2197cecdb02c ("sched_ext: idle: Prioritize idle SMT sibling") Reported-by: kernel test robot <lkp@intel.com> Closes: https://lore.kernel.org/oe-kbuild-all/202603221422.XIueJOE9-lkp@intel.com/ Signed-off-by: Andrea Righi <arighi@nvidia.com> Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com> Signed-off-by: Tejun Heo <tj@kernel.org> Suggested-by: Wentao Guan <guanwentao@uniontech.com> Signed-off-by: WangYuli <wangyl5933@chinaunicom.cn>

Avenger-285714 · 2026-05-18T11:47:14Z

commit 63f500c32a37d490ec623a3130e488cdb9bd6cf7 Author: Andrea Righi arighi@nvidia.com Date: Sun Mar 22 07:51:46 2026 +0100

sched_ext: Guard cpu_smt_mask() with CONFIG_SCHED_SMT

Wrap cpu_smt_mask() usage with CONFIG_SCHED_SMT to avoid build failures
on kernels built without SMT support.

Fixes: 2197cecdb02c ("sched_ext: idle: Prioritize idle SMT sibling")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603221422.XIueJOE9-lkp@intel.com/
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

done

Avenger-285714 requested review from Copilot and opsiff May 18, 2026 09:24

deepin-ci-robot requested review from huangbibo and myml May 18, 2026 09:24

Avenger-285714 changed the title ~~sched_ext: idle: Prioritize idle SMT sibling~~ [Deepin-Kernel-SIG] [linux 6.18.y] [Upstream] sched_ext: idle: Prioritize idle SMT sibling May 18, 2026

Copilot started reviewing on behalf of Avenger-285714 May 18, 2026 09:26 View session

sourcery-ai Bot reviewed May 18, 2026

View reviewed changes

Copilot AI reviewed May 18, 2026

View reviewed changes

Comment thread kernel/sched/ext_idle.c

Comment on lines +625 to +635

/*

* Use @prev_cpu's sibling if it's idle.

*/

if (sched_smt_active()) {

for_each_cpu_and(cpu, cpu_smt_mask(prev_cpu), allowed) {

if (cpu == prev_cpu)

continue;

if (scx_idle_test_and_clear_cpu(cpu))

goto out_unlock;

}

}

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

[Deepin-Kernel-SIG] [linux 6.18.y] [Upstream] sched_ext: idle: Prioritize idle SMT sibling#1743

[Deepin-Kernel-SIG] [linux 6.18.y] [Upstream] sched_ext: idle: Prioritize idle SMT sibling#1743
Avenger-285714 wants to merge 2 commits into
deepin-community:linux-6.18.yfrom
Avenger-285714:Prioritize-SMT

Avenger-285714 commented May 18, 2026 •

edited by sourcery-ai Bot

Loading

Uh oh!

sourcery-ai Bot commented May 18, 2026 •

edited

Loading

Interacting with Sourcery

Customizing Your Experience

Getting Help

Uh oh!

deepin-ci-robot commented May 18, 2026

Uh oh!

sourcery-ai Bot left a comment

Uh oh!

Copilot AI left a comment

Uh oh!

opsiff commented May 18, 2026

Uh oh!

Avenger-285714 commented May 18, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Conversation

Avenger-285714 commented May 18, 2026 • edited by sourcery-ai Bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary by Sourcery

Uh oh!

sourcery-ai Bot commented May 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Reviewer's Guide

File-Level Changes

Interacting with Sourcery

Customizing Your Experience

Getting Help

Uh oh!

deepin-ci-robot commented May 18, 2026

Uh oh!

sourcery-ai Bot left a comment

Choose a reason for hiding this comment

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

opsiff commented May 18, 2026

Uh oh!

Avenger-285714 commented May 18, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Avenger-285714 commented May 18, 2026 •

edited by sourcery-ai Bot

Loading

sourcery-ai Bot commented May 18, 2026 •

edited

Loading