ct/l1: add cloud_topics_compaction scheduling group

[WillemKauf]

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

• none

[Copilot]

Pull request overview

This PR adds a new cloud_topics_compaction scheduling group to better isolate CPU resources for cloud topics compaction operations. The change introduces a dedicated scheduling group with a share count of 150, adds the necessary accessor method, and integrates it into the cloud topics level one compaction worker.

Changes:

• Added a new cloud_topics_compaction scheduling group with 150 shares in the CPU scheduling system
• Updated the compaction worker to accept and use the new scheduling group for its work loop
• Updated test fixtures to accommodate the new scheduling group parameter

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/v/resource_mgmt/cpu_scheduling.h	Defines and initializes the new cloud_topics_compaction scheduling group
src/v/cloud_topics/level_one/compaction/worker_manager.cc	Passes the new scheduling group to worker initialization
src/v/cloud_topics/level_one/compaction/worker.h	Adds scheduling group parameter to constructor and member variable
src/v/cloud_topics/level_one/compaction/worker.cc	Stores the scheduling group and wraps work loop execution with it
src/v/cloud_topics/level_one/compaction/tests/worker_manager_test.cc	Updates test to provide default scheduling group
src/v/cloud_topics/level_one/compaction/tests/scheduler_fixture.h	Updates test fixture to provide default scheduling group
src/v/cloud_topics/level_one/compaction/BUILD	Adds dependency on cpu_scheduling library

[WillemKauf]

Oops. Looks like we're capping out at 18 scheduling groups max, and cloud_topics_compaction kicks us over the edge:

redpanda/bazel/thirdparty/seastar.BUILD

Lines 75 to 79 in 2a9fc3b

	int_flag(
	name = "scheduling_groups",
	build_setting_default = 18,
	make_variable = "SCHEDULING_GROUPS",
	)

Is there any reason we can't kick the can down the road by bumping this? We should also probably start investigating use of scheduling supergroups:

https://github.com/scylladb/seastar/blob/2a537bf22e7db2157b8d709313d2b9462ef2b83e/include/seastar/core/scheduling.hh#L64-L72

[dotnwat]

whats the status on this are we planning to make this change?

[WillemKauf]

whats the status on this are we planning to make this change?

🤷

there was a brief discussion on scheduling groups here. Probably something to raise again with the team in a meeting.

[WillemKauf]

Closing in favour of re-using the existing compaction scheduling group here: #29492

[wdberkeley]

Maybe I missed a discussion on this, but should we give ourselves headroom so this doesn't need to get bumped each time we add a scheduling group? Like we jump to 24 or 32?

[WillemKauf]

My thinking was to leave it maxed out just so people really have to think about adding more before they do so.

[vbotbuildovich]

CI test results

test results on build#80107

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
QuotaManagementUpgradeTest	test_upgrade	null	integration	https://buildkite.com/redpanda/redpanda/builds/80107#019c29ee-d693-4880-a37f-bb6fdaac98c6	FLAKY	9/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0674, p0=0.5026, reject_threshold=0.0100. adj_baseline=0.1890, p1=0.4099, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=QuotaManagementUpgradeTest&test_method=test_upgrade