Re: [Xen-devel] [PATCH] xen: do live patching only from main idle loop

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Juergen Gross <jgross@xxxxxxxx>
> Sent: Tuesday, February 11, 2020 5:31 PM
> 
> One of the main design goals of core scheduling is to avoid actions
> which are not directly related to the domain currently running on a
> given cpu or core. Live patching is one of those actions which are
> allowed taking place on a cpu only when the idle scheduling unit is
> active on that cpu.
> 
> Unfortunately live patching tries to force the cpus into the idle loop
> just by raising the schedule softirq, which will no longer be
> guaranteed to work with core scheduling active. Additionally there are
> still some places in the hypervisor calling check_for_livepatch_work()
> without being in the idle loop.
> 
> It is easy to force a cpu into the main idle loop by scheduling a
> tasklet on it. So switch live patching to use tasklets for switching to
> idle and raising scheduling events. Additionally the calls of
> check_for_livepatch_work() outside the main idle loop can be dropped.
> 
> As tasklets are only running on idle vcpus and stop_machine_run()
> is activating tasklets on all cpus but the one it has been called on
> to rendezvous, it is mandatory for stop_machine_run() to be called on
> an idle vcpu, too, as otherwise there is no way for scheduling to
> activate the idle vcpu for the tasklet on the sibling of the cpu
> stop_machine_run() has been called on.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

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