Re: profiling virtio (blk in this case)

[Avi Kivity <avi@redhat.com>]

Eran Rom wrote:
> The "Host<-->Guest" calls in virtio_blk (using the generic virtio kick/notify)
> are as follows:
>
> Guest->Host
> -----------
> do_virtlkb_request calls kick in the guest side causing handle_output to be
> called on the host side.
>
> Host->Guest
> -----------
> virtio_blk_rw_complete calls notify in the host side causing block_done to be
> called on the guest side
>
> My question has to do with the timing of the calls.
> Which would be the correct drawing
> 1. Overlapping:
> kick             |------------|
> handle_output             |------------|
> 2. Disjoint:
> kick             |------------|
> handle_output                    |------------|
>   

kick() competely contains handle_output().

> In other words:
> if I do
> g1 = get_cpu_cycles
> kick
> g2 = get_cpu_cycles
> and 
> h1 = get_cpu_cycles
> handle_output
> h2 = get_cpu_cycles
> would (g2-g1) + (h2-h2) count some cycles twice?
>   

h2-h1 would be counted twice.

> Same question for notify and block_done.
>   

Different answer.  The guest starts executing sometimes in the middle of 
the host notify function.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.