Re: [Xen-devel] Scheduling and the periodic timer

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.02.19 at 08:26, <jgross@xxxxxxxx> wrote:
> While working on my core scheduling series I stumbled over the periodic
> timer. Could it be this timer never worked correctly?
> 
> When the vcpu with an active periodic timer is running everything seems
> to be fine. But when not running the timer is stopped in schedule(). So
> a vcpu going to idle relying to be woken up by the periodic timer will
> remain sleeping until another event is happening. The periodic timer
> won't fire as it is stopped.
> 
> The periodic timer is used via VCPUOP_set_periodic_timer only, and
> today's Linux kernel isn't using it at all. So I guess this timer not
> really working as it should is no big issue.

Interesting. Our kernels have been using it until SLE11 SP1, as
did the old 2.6.18 one that all of our old kernels were derived
from. Yet the same code is present already in 3.2.0's schedule().
Are you sure this timer is meant to wake the vCPU at the set
rate, rather than just surfacing events if the vCPU is running?
Looking at vcpu_periodic_timer_work()'s calculation of the
next event time also suggests to me that there's no guarantee
that the event will indeed surface at the set rate.

Jan



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