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Re: [Xen-devel] [PATCH v2 13/48] xen/sched: add is_running indicator to struct sched_unit



On 11.09.19 17:06, Jan Beulich wrote:
On 11.09.2019 15:44, Juergen Gross wrote:
On 04.09.19 17:06, Jan Beulich wrote:
On 09.08.2019 16:57, Juergen Gross wrote:
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -407,6 +407,8 @@ int sched_init_vcpu(struct vcpu *v, unsigned int processor)
       {
           get_sched_res(v->processor)->curr = unit;
           v->is_running = 1;
+        unit->is_running = 1;
+        unit->state_entry_time = NOW();
       }
       else
       {

Are idle vCPU-s also going to get grouped into units (I'm sorry,
I don't recall)? If so, just like further down I'd be putting
under question the multiple writing of the field. I'd kind of
expect the unit and all vCPU-s within it to get an identical
state entry time stored.

When an idle vcpu is being created its cpu is in no cpupool yet
(a free cpu). Those cpus are always subject to cpu scheduling
(granularity 1). Only when cpus are put into a cpupool the
granularity is adjusted accordingly and the idle-vcpus are
possibly grouped in units (see patch 45).

Regarding the state entry time: different vcpus of a unit can be
in different states (blocked/running, etc.), so their state entry
times will generally differ. A unit's state entry time will
reflect its scheduling events (being scheduled/descheduled).

But this doesn't (to me) clarify whether the multiple writing
(once per vCPU) is actually correct. Obviously whether this is
merely cosmetic depends on what the consumers of this data are.

There is no multiple writing. This is a preparatory patch for being
able to remove the vcpu dependencies from the single schedulers.

Later when multiple vcpus per unit are introduced there will be a
clear distinction between vcpu switching (context switches) and
sched_unit switching (scheduling events). vcpu data will be adjusted
at context_switch time, while unit data will be adjusted only when
scheduling units.


Also both here and further down I'm puzzled to see that the
unit's field doesn't get set at the same place in code where
the vCPU's field gets set.

Right. The states of a vcpu and the unit it is belonging to are
coupled, but not identical.

And this is not (going to be) a problem?

With a proper distinction as explained above, no.


Juergen


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