Re: [Xen-devel] [PATCH v2 35/48] xen/sched: add fall back to idle vcpu when scheduling unit

[Julien Grall <julien.grall@xxxxxxx>]

Hi Juergen,

On 8/9/19 3:58 PM, Juergen Gross wrote:
> When scheduling an unit with multiple vcpus there is no guarantee all
> vcpus are available (e.g. above maxvcpus or vcpu offline). Fall back to
> idle vcpu of the current cpu in that case. This requires to store the
> correct schedule_unit pointer in the idle vcpu as long as it used as
> fallback vcpu.
>
> In order to modify the runstates of the correct vcpus when switching
> schedule units merge sched_unit_runstate_change() into
> sched_switch_units() and loop over the affected physical cpus instead
> of the unit's vcpus. This in turn requires an access function to the
> current variable of other cpus.
>
> Today context_saved() is called in case previous and next vcpus differ
> when doing a context switch. With an idle vcpu being capable to be a
> substitute for an offline vcpu this is problematic when switching to
> an idle scheduling unit. An idle previous vcpu leaves us in doubt which
> schedule unit was active previously, so save the previous unit pointer
> in the per-schedule resource area and use its value being non-NULL as
> a hint whether context_saved() should be called.
>
> When running an idle vcpu in a non-idle scheduling unit use a specific
> guest idle loop not performing any tasklets and livepatching in order
> to avoid populating the cpu caches with memory used by other domains
> (as far as possible). Softirqs are considered to be save.
>
> In order to avoid livepatching when going to guest idle another
> variant of reset_stack_and_jump() not calling check_for_livepatch_work
> is needed.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
I haven't looked at the full patch. The arm change is quite minimal, so 
for just arm-bits:
Acked-by: Julien Grall <julien.grall@xxxxxxx>

Cheers,

--
Julien Grall

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