----- On Oct 17, 2018, at 3:19 AM, Srikar Dronamraju srikar@linux.vnet.ibm.com wrote:

Hi Mathieu,

quoted

+static int do_cpu_opv(struct cpu_op *cpuop, int cpuopcnt,
+		      struct cpu_opv_vaddr *vaddr_ptrs, int cpu)
+{
+	struct mm_struct *mm = current->mm;
+	int ret;
+
+retry:
+	if (cpu != raw_smp_processor_id()) {
+		ret = push_task_to_cpu(current, cpu);
+		if (ret)
+			goto check_online;
+	}
+	down_read(&mm->mmap_sem);
+	ret = vaddr_ptrs_check(vaddr_ptrs);
+	if (ret)
+		goto end;
+	preempt_disable();
+	if (cpu != smp_processor_id()) {
+		preempt_enable();
+		up_read(&mm->mmap_sem);
+		goto retry;
+	}

If we have a higher priority task/s either pinned to the cpu, dont we end up
in busy-looping till the task exits/sleeps?

You're right!

How about we ditch the thread migration altogether, and simply perform
the cpu_opv operations in a IPI handler ?

This is possible now that cpu_opv uses a temporary vmap() rather than
try to touch the user-space page through the current thread's page table.

Thoughts ?

Thanks,

Mathieu

quoted

+	ret = __do_cpu_opv(cpuop, cpuopcnt);
+	preempt_enable();
+end:
+	up_read(&mm->mmap_sem);
+	return ret;
+
+check_online:
+	/*
+	 * push_task_to_cpu() returns -EINVAL if the requested cpu is not part
+	 * of the current thread's cpus_allowed mask.
+	 */
+	if (ret == -EINVAL)
+		return ret;
+	get_online_cpus();
+	if (cpu_online(cpu)) {
+		put_online_cpus();
+		goto retry;
+	}

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com