Hello Shrikanth,

On 9/10/2025 11:12 PM, Shrikanth Hegde wrote:

Actively push out any task running on a paravirt CPU. Since the task is
running on the CPU need to spawn a stopper thread and push the task out.

If task is sleeping, when it wakes up it is expected to move out. In
case it still chooses a paravirt CPU, next tick will move it out.
However, if the task in pinned only to paravirt CPUs, it will continue
running there.

Though code is almost same as __balance_push_cpu_stop and quite close to
push_cpu_stop, it provides a cleaner implementation w.r.t to PARAVIRT
config.

Add push_task_work_done flag to protect pv_push_task_work buffer. This has
been placed at the empty slot available considering 64/128 byte
cacheline.

This currently works only FAIR and RT.

EXT can perhaps use the ops->cpu_{release,acquire}() if they are
interested in this.

Does this mean paravirt CPU is capable of handling an interrupt but may
not be continuously available to run a task? Or is the VMM expected to set
the CPU on the paravirt mask and give the vCPU sufficient time to move the
task before yanking it away from the pCPU?

Can we move all pushable tasks at once instead of just the rq->curr at
the time of the tick? It can also avoid keeping the reference to "p"
and only selectively pushing it. Thoughts?

+	struct rq *rq = this_rq();
+	struct rq_flags rf;
+	int cpu;
+
+	raw_spin_lock_irq(&p->pi_lock);
+	rq_lock(rq, &rf);
+	rq->push_task_work_done = 0;
+
+	update_rq_clock(rq);
+
+	if (task_rq(p) == rq && task_on_rq_queued(p)) {
+		cpu = select_fallback_rq(rq->cpu, p);
+		rq = __migrate_task(rq, &rf, p, cpu);
+	}
+
+	rq_unlock(rq, &rf);
+	raw_spin_unlock_irq(&p->pi_lock);
+	put_task_struct(p);
+
+	return 0;
+}
+
+/* A CPU is marked as Paravirt when there is contention for underlying
+ * physical CPU and using this CPU will lead to hypervisor preemptions.
+ * It is better not to use this CPU.
+ *
+ * In case any task is scheduled on such CPU, move it out. In
+ * select_fallback_rq a non paravirt CPU will be chosen and henceforth
+ * task shouldn't come back to this CPU
+ */
+void push_current_from_paravirt_cpu(struct rq *rq)
+{
+	struct task_struct *push_task = rq->curr;
+	unsigned long flags;
+	struct rq_flags rf;
+
+	if (!is_cpu_paravirt(rq->cpu))
+		return;
+
+	/* Idle task can't be pused out */
+	if (rq->curr == rq->idle)
+		return;
+
+	/* Do for only SCHED_NORMAL AND RT for now */
+	if (push_task->sched_class != &fair_sched_class &&
+	    push_task->sched_class != &rt_sched_class)
+		return;
+
+	if (kthread_is_per_cpu(push_task) ||
+	    is_migration_disabled(push_task))
+		return;
+
+	/* Is it affine to only paravirt cpus? */
+	if (cpumask_subset(push_task->cpus_ptr, cpu_paravirt_mask))
+		return;
+
+	/* There is already a stopper thread for this. Dont race with it */
+	if (rq->push_task_work_done == 1)
+		return;
+
+	local_irq_save(flags);
+	preempt_disable();

Disabling IRQs implies preemption is disabled.

+
+	get_task_struct(push_task);
+
+	rq_lock(rq, &rf);
+	rq->push_task_work_done = 1;
+	rq_unlock(rq, &rf);
+
+	stop_one_cpu_nowait(rq->cpu, paravirt_push_cpu_stop, push_task,
+			    this_cpu_ptr(&pv_push_task_work));
+	preempt_enable();
+	local_irq_restore(flags);
+}
 #endif

-- 
Thanks and Regards,
Prateek