Re: [PATCH/RFC] KVM: track pid for VCPU only on KVM_RUN ioctl

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 08/05/2014 08:14 PM, Christian Borntraeger wrote:
> We currently track the pid of the task that runs the VCPU in
> vcpu_load. Since we call vcpu_load for all kind of ioctls on a
> CPU, this causes hickups due to synchronize_rcu if one CPU is
> modified by another CPU or the main thread (e.g. initialization,
> reset). We track the pid only for the purpose of yielding, so
> let's update the pid only in the KVM_RUN ioctl.
>
> In addition, don't do a synchronize_rcu on startup (pid == 0).
>
> This speeds up guest boot time on s390 noticably for some configs, e.g.
> HZ=100, no full state tracking, 64 guest cpus 32 host cpus.
>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> CC: Rik van Riel <riel@redhat.com>
> CC: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> CC: Michael Mueller <mimu@linux.vnet.ibm.com>
> ---

Please feel free to add
Reviewed-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>

I could see very small improvement while testing 32 vcpu guest booting
on x86 (16 pcpu host +ht).

I was just wondering whether somebody implementing vcpu hot plug would
have to bother about this change, but could not see any. What do you
think?