Re: [PATCH] trace-cmd: rework of the pid detection of vcpus

[Vineeth Pillai <vineethrp@google.com>]

Hi Steven,

On Mon, Jul 11, 2022 at 3:16 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> If you haven't noticed I have patches that are a workaround that fixes this
> issue if the conditions are "right". (I Cc'd you) [1]
I was on vacation and just combing through the mails :-). I shall have a look
at the patch soon..

>
> The work around is to not depend on the number in debugfs/kvm/vcpuX to
> match the CPU of the guest, but instead to match the ordering of the CPUs
> of the guest. It also requires that the guest's CPUs are consecutive and
> non sparse. That is, if you have vcpu0, vcpu1, vcpu8 and vcpu9 on the
> host, then the guest needs to have CPU 0-3 with the mapping of:
>
>  vcpu0 -> CPU 0
>  vcpu1 -> CPU 1
>  vcpu8 -> CPU 2
>  vcpu9 -> CPU 3
>
> and then it works. Of course if this is not true, then it breaks again and
> will require some more communication between the host and the guest.
This makes sense.

I vaguely remember seeing non-increasing patterns in some server machines,
but that's really rare I guess. When the debugfs changes for exposing pid are
widely available, we can have that also as an option.

Thanks for the patches. I shall have a look at the patches soon.

~Vineeth