Re: [Xen-devel] [PATCH v2 1/2] xen: sched: don't call hooks of the wrong scheduler via VCPU2OP

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.03.17 at 19:29, <jgross@xxxxxxxx> wrote:
> On 17/03/17 19:19, Dario Faggioli wrote:
>> Within context_saved(), we call the context_saved hook,
>> and we use VCPU2OP() to determine from what scheduler.
>> VCPU2OP uses DOM2OP, which uses d->cpupool, which is
>> NULL when d is the idle domain. And in that case,
>> DOM2OP just returns ops, the scheduler of cpupool0.
>> 
>> Therefore, if:
>> - cpupool0's scheduler defines context_saved (like
>>   Credit2 and RTDS do),
>> - we are not in cpupool0 (i.e., our scheduler is
>>   not ops),
>> - we are context switching from idle,
>> 
>> we call VCPU2OP(idle_vcpu), which means
>> DOM2OP(idle->cpupool), which is ops.
>> 
>> Therefore, we both:
>> - check if context_saved is defined in the wrong
>>   scheduler;
>> - if yes, call the wrong one.
>> 
>> When using Credit2 at boot, and also Credit2 in
>> the other cpupool, this is wrong but innocuous,
>> because it only involves the idle vcpus.
>> 
>> When using Credit2 at boot, and Credit1 in the
>> other cpupool, this is *totally* wrong, and
>> it's by chance it does not explode!
>> 
>> When using Credit2 and other schedulers I'm
>> developping, I hit the following assert (in
>> sched_credit2.c, on a CPU inside a cpupool that
>> does not use Credit2):
>> 
>> csched2_context_saved()
>> {
>>  ...
>>  ASSERT(!vcpu_on_runq(svc));
>>  ...
>> }
>> 
>> Fix this by dealing explicitly, in VCPU2OP, with
>> idle vcpus, returning the scheduler of the pCPU
>> they (always) run on.
>> 
>> While there, rename VCPU2OP itself to something
>> that makes it easier to understand what it does.
>> 
>> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> 
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

This came through twice, so I'm now wondering whether one of
them was meant for 2/2, or whether this was just some
infrastructure glitch.

Jan


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