Re: [PATCH 08/13] migration/postcopy: Report fault latencies in blocktime

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> Blocktime so far only cares about the time one vcpu (or the whole system)
> got blocked.  It would be also be helpful if it can also report the latency
> of page requests, which could be very sensitive during postcopy.
>
> Blocktime itself is sometimes not very important, especially when one
> thinks about KVM async PF support, which means vCPUs are literally almost
> not blocked at all because the guest OS is smart enough to switch to
> another task when a remote fault is needed.
>
> However, latency is still sensitive and important because even if the guest
> vCPU is running on threads that do not need a remote fault, the workload
> that accesses some missing page is still affected.
>
> Add two entries to the report, showing how long it takes to resolve a
> remote fault.  Mention in the QAPI doc that this is not the real average
> fault latency, but only the ones that was requested for a remote fault.
>
> Unwrap get_vcpu_blocktime_list() so we don't need to walk the list twice,
> meanwhile add the entry checks in qtests for all postcopy tests.
>
> Cc: Markus Armbruster <armbru@redhat.com>
> Cc: Dr. David Alan Gilbert <dave@treblig.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>