Re: [PATCH] kvm: reuse per-vcpu stats fd to avoid vcpu interruption

[Markus Armbruster <armbru@redhat.com>]

Marcelo Tosatti <mtosatti@redhat.com> writes:

> A regression has been detected in latency testing of KVM guests.
> More specifically, it was observed that the cyclictest
> numbers inside of an isolated vcpu (running on isolated pcpu) are:
>
> # Max Latencies: 00090 00096 00141
>
> Where a maximum of 50us is acceptable.
>
> The implementation of KVM_GET_STATS_FD uses run_on_cpu to query
> per vcpu statistics, which interrupts the vcpu (and is unnecessary).
>
> To fix this, open the per vcpu stats fd on vcpu initialization,
> and read from that fd from QEMU's main thread.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

[...]

> @@ -4038,7 +4040,8 @@ static StatsDescriptors *find_stats_descriptors(StatsTarget target, int stats_fd
>  }
>  
>  static void query_stats(StatsResultList **result, StatsTarget target,
> -                        strList *names, int stats_fd, Error **errp)
> +                        strList *names, int stats_fd, Error **errp,
> +                        CPUState *cpu)

include/qapi/error.h:

 * - Functions that use Error to report errors have an Error **errp
 *   parameter.  It should be the last parameter, except for functions
 *   taking variable arguments.

>  {
>      struct kvm_stats_desc *kvm_stats_desc;
>      struct kvm_stats_header *kvm_stats_header;

[...]