* Question on clock drift with NO_HZ_FULL vs NO_HZ_IDLE
@ 2014-02-23 7:20 Joel Fernandes
2014-02-23 16:38 ` Paul E. McKenney
0 siblings, 1 reply; 3+ messages in thread
From: Joel Fernandes @ 2014-02-23 7:20 UTC (permalink / raw)
To: linux-rt-users; +Cc: Paul E. McKenney
Hi,
After reading documentation, I follow that NO_HZ_FULL turns off the
scheduling-clock interrupt on all except the boot CPU, where as
NO_HZ_IDLE turns it off on all idle CPUs.
I understand the period scheduling-clock interrupt is required to
compensate for hardware oscillator drift. I believe that's why
NO_HZ_FULL keeps it ON on atleast one CPU. That way the drift is
compensated for.
But, with NO_HZ_IDLE, all CPUs turn off interrupt during idle. Then
how is the drift accounted for?
CC'ing Paul as well for any guidance, thanks :)
Regards,
-Joel
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Question on clock drift with NO_HZ_FULL vs NO_HZ_IDLE
2014-02-23 7:20 Question on clock drift with NO_HZ_FULL vs NO_HZ_IDLE Joel Fernandes
@ 2014-02-23 16:38 ` Paul E. McKenney
2014-02-23 17:22 ` Joel Fernandes
0 siblings, 1 reply; 3+ messages in thread
From: Paul E. McKenney @ 2014-02-23 16:38 UTC (permalink / raw)
To: Joel Fernandes; +Cc: linux-rt-users, fweisbec
On Sun, Feb 23, 2014 at 01:20:53AM -0600, Joel Fernandes wrote:
> Hi,
> After reading documentation, I follow that NO_HZ_FULL turns off the
> scheduling-clock interrupt on all except the boot CPU, where as
> NO_HZ_IDLE turns it off on all idle CPUs.
>
> I understand the period scheduling-clock interrupt is required to
> compensate for hardware oscillator drift. I believe that's why
> NO_HZ_FULL keeps it ON on atleast one CPU. That way the drift is
> compensated for.
>
> But, with NO_HZ_IDLE, all CPUs turn off interrupt during idle. Then
> how is the drift accounted for?
>
> CC'ing Paul as well for any guidance, thanks :)
The clock code fixes things up as needed when the first CPU returns
from idle.
Thanx, Paul
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Question on clock drift with NO_HZ_FULL vs NO_HZ_IDLE
2014-02-23 16:38 ` Paul E. McKenney
@ 2014-02-23 17:22 ` Joel Fernandes
0 siblings, 0 replies; 3+ messages in thread
From: Joel Fernandes @ 2014-02-23 17:22 UTC (permalink / raw)
To: paulmck; +Cc: linux-rt-users, fweisbec
Hi Paul,
On Sun, Feb 23, 2014 at 10:38 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Sun, Feb 23, 2014 at 01:20:53AM -0600, Joel Fernandes wrote:
>> Hi,
>> After reading documentation, I follow that NO_HZ_FULL turns off the
>> scheduling-clock interrupt on all except the boot CPU, where as
>> NO_HZ_IDLE turns it off on all idle CPUs.
>>
>> I understand the period scheduling-clock interrupt is required to
>> compensate for hardware oscillator drift. I believe that's why
>> NO_HZ_FULL keeps it ON on atleast one CPU. That way the drift is
>> compensated for.
>>
>> But, with NO_HZ_IDLE, all CPUs turn off interrupt during idle. Then
>> how is the drift accounted for?
>>
>> CC'ing Paul as well for any guidance, thanks :)
>
> The clock code fixes things up as needed when the first CPU returns
> from idle.
Thanks a lot for your reply. It makes sense to me now. Also, thanks
for the great talk at LCA. :)
regards,
-Joel
^ permalink raw reply [flat|nested] 3+ messages in thread
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2014-02-23 7:20 Question on clock drift with NO_HZ_FULL vs NO_HZ_IDLE Joel Fernandes
2014-02-23 16:38 ` Paul E. McKenney
2014-02-23 17:22 ` Joel Fernandes
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