Re: [PATCH 1/2] sched: Feature to decide if steal should update CPU capacity

* Peter Zijlstra [off-list ref] [2025-10-28 12:18:13]:

On Tue, Oct 28, 2025 at 04:12:54PM +0530, Srikar Dronamraju wrote:

quoted

At present, scheduler scales CPU capacity for fair tasks based on time
spent on irq and steal time. If a CPU sees irq or steal time, its
capacity for fair tasks decreases causing tasks to migrate to other CPU
that are not affected by irq and steal time. All of this is gated by
NONTASK_CAPACITY.

In virtualized setups, a CPU that reports steal time (time taken by the
hypervisor) can cause tasks to migrate unnecessarily to sibling CPUs that
appear to be less busy, only for the situation to reverse shortly.

To mitigate this ping-pong behaviour, this change introduces a new
scheduler feature flag: ACCT_STEAL which will control whether steal time
contributes to non-task capacity adjustments (used for fair scheduling).

Please don't use sched_feat like this. If this is something that wants
to be set by architectures move it to a normal static_branch (like eg.
sched_energy_present, sched_asymc_cpucapacity, sched_cluster_active,
sched_smt_present, sched_numa_balancing etc.).

Ok, Peter, will move it to a static_branch approach and post a v2.

-- 
Thanks and Regards
Srikar Dronamraju