Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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

> Hi Mathieu,
> 
>> +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

> 
>> +	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