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

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