[PATCH 08/17] sched/core: Implement CPU soft offline/online
From: Srikar Dronamraju <hidden>
Date: 2025-12-04 17:56:20
Also in:
lkml, virtualization
Subsystem:
scheduler, the rest · Maintainers:
Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Linus Torvalds
Scheduler already supports CPU online/offline. However for cases where scheduler has to offline a CPU temporarily, the online/offline cost is too high. Hence here is an attempt to come-up with soft-offline that almost looks similar to offline without actually having to do the full-offline. Since CPUs are not to be used temporarily for a short duration, they will continue to be part of the CPU topology. In the soft-offline, CPU will be marked as inactive, i.e removed from the cpu_active_mask, CPUs capacity would be reduced and non-pinned tasks would be migrated out of the CPU's runqueue. Similarly when onlined, CPU will be remarked as active, i.e. added to cpu_active_mask, CPUs capacity would be restored. Soft-offline is almost similar as 1st step of offline except rebuilding the sched-domains. Since the other steps are not done including rebuilding the sched-domain, the overhead of soft-offline would be less compared to regular offline. A new cpumask is used to indicate soft-offline is in progress and hence skips rebuilding the sched-domains. To push tasks out of the CPU, balance_push is modified to push tasks out till there are runnable tasks on the runqueue or till the CPU is in dying state. Signed-off-by: Srikar Dronamraju <redacted> --- include/linux/sched/topology.h | 1 + kernel/sched/core.c | 44 ++++++++++++++++++++++++++++++---- 2 files changed, 40 insertions(+), 5 deletions(-)
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index bbcfdf12aa6e..ed45d7db3e76 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h@@ -241,4 +241,5 @@ static inline int task_node(const struct task_struct *p) return cpu_to_node(task_cpu(p)); } +extern void set_cpu_softoffline(int cpu, bool soft_offline); #endif /* _LINUX_SCHED_TOPOLOGY_H */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 89efff1e1ead..f66fd1e925b0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c@@ -8177,13 +8177,16 @@ static void balance_push(struct rq *rq) * Only active while going offline and when invoked on the outgoing * CPU. */ - if (!cpu_dying(rq->cpu) || rq != this_rq()) + if (cpu_active(rq->cpu) || rq != this_rq()) return; /* - * Ensure the thing is persistent until balance_push_set(.on = false); + * Unless soft-offline, Ensure the thing is persistent until + * balance_push_set(.on = false); In case of soft-offline, just + * enough to push current non-pinned tasks out. */ - rq->balance_callback = &balance_push_callback; + if (cpu_dying(rq->cpu) || rq->nr_running) + rq->balance_callback = &balance_push_callback; /* * Both the cpu-hotplug and stop task are in this case and are
@@ -8392,6 +8395,8 @@ static inline void sched_smt_present_dec(int cpu) #endif } +static struct cpumask cpu_softoffline_mask; + int sched_cpu_activate(unsigned int cpu) { struct rq *rq = cpu_rq(cpu);
@@ -8411,7 +8416,10 @@ int sched_cpu_activate(unsigned int cpu) if (sched_smp_initialized) { sched_update_numa(cpu, true); sched_domains_numa_masks_set(cpu); - cpuset_cpu_active(); + + /* For CPU soft-offline, dont need to rebuild sched-domains */ + if (!cpumask_test_cpu(cpu, &cpu_softoffline_mask)) + cpuset_cpu_active(); } scx_rq_activate(rq);
@@ -8485,7 +8493,11 @@ int sched_cpu_deactivate(unsigned int cpu) return 0; sched_update_numa(cpu, false); - cpuset_cpu_inactive(cpu); + + /* For CPU soft-offline, dont need to rebuild sched-domains */ + if (!cpumask_test_cpu(cpu, &cpu_softoffline_mask)) + cpuset_cpu_inactive(cpu); + sched_domains_numa_masks_clear(cpu); return 0; }
@@ -10928,3 +10940,25 @@ void sched_enq_and_set_task(struct sched_enq_and_set_ctx *ctx) set_next_task(rq, ctx->p); } #endif /* CONFIG_SCHED_CLASS_EXT */ + +void set_cpu_softoffline(int cpu, bool soft_offline) +{ + struct sched_domain *sd; + + if (!cpu_online(cpu)) + return; + + cpumask_set_cpu(cpu, &cpu_softoffline_mask); + + rcu_read_lock(); + for_each_domain(cpu, sd) + update_group_capacity(sd, cpu); + rcu_read_unlock(); + + if (soft_offline) + sched_cpu_deactivate(cpu); + else + sched_cpu_activate(cpu); + + cpumask_clear_cpu(cpu, &cpu_softoffline_mask); +}
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2.43.7