[RFC 04/11] powerpc: kvm: introduce a kthread on primary thread to anti tickless

[Liu ping fan]

On Mon, Oct 27, 2014 at 12:45 PM, Preeti U Murthy
<preeti at linux.vnet.ibm.com> wrote:
> On 10/17/2014 12:59 AM, kernelfans at gmail.com wrote:
>> (This patch is a place holder.)
>>
>> If there is only one vcpu thread is ready(the other vcpu thread can
>> wait for it to execute), the primary thread can enter tickless mode,
>
> We do not configure NOHZ_FULL to y by default. Hence no thread would
> enter tickless mode.
>
But NOHZ_FULL can be chosen by user, and we should survive from it :)

>> which causes the primary keeps running, so the secondary has no
>> opportunity to exit to host, even they have other tsk on them.
>
> The secondary threads can still get scheduling ticks. The decrementer of
> the secondary threads is still active. So as long as secondary threads
> are busy, scheduling ticks will fire and try to schedule a new task on them.
>
No. As my original thought, after enable KVM on core, the HDEC on
secondary is disabled, otherwise the host exit will be too frequent.
Any suggestion?

Thx,
Fan

> Regards
> Preeti U Murthy
>>
>> Introduce a kthread (anti_tickless) on primary, so when there is only
>> one vcpu thread on primary, the secondary can resort to anti_tickless
>> to keep the primary out of tickless mode.
>> (I thought that anti_tickless thread can goto NAP, so we can let the
>> secondary run).
>>
>> Signed-off-by: Liu Ping Fan <pingfank at linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kernel/sysfs.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
>> index a2595dd..f0b110e 100644
>> --- a/arch/powerpc/kernel/sysfs.c
>> +++ b/arch/powerpc/kernel/sysfs.c
>> @@ -575,9 +575,11 @@ static ssize_t __used store_kvm_enable(struct device *dev,
>>       if (!test_bit(core, &kvm_on_core))
>>               for (thr = 1; thr< threads_per_core; thr++)
>>                       if (cpu_online(thr * threads_per_core + thr))
>> -                             cpumask_set_cpu(thr * threads_per_core + thr, &stop_cpus);
>> +                             cpumask_set_cpu(core * threads_per_core + thr, &stop_cpus);
>>
>>       stop_machine(xics_migrate_irqs_away_secondary, NULL, &stop_cpus);
>> +     /* fixme, create a kthread on primary hwthread to handle tickless mode */
>> +     //kthread_create_on_cpu(prevent_tickless, NULL, core * threads_per_core, "ppckvm_prevent_tickless");
>>       set_bit(core, &kvm_on_core);
>>       return count;
>>  }
>>
>