linux-kernel - Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

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Date:   Thu, 13 Dec 2018 15:12:29 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] vhost: accelerate metadata access through
 vmap()

On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote:
> Hi:
> 
> This series tries to access virtqueue metadata through kernel virtual
> address instead of copy_user() friends since they had too much
> overheads like checks, spec barriers or even hardware feature
> toggling.
> 
> Test shows about 24% improvement on TX PPS. It should benefit other
> cases as well.
> 
> Please review

I think the idea of speeding up userspace access is a good one.
However I think that moving all checks to start is way too aggressive.
Instead, let's batch things up but let's not keep them
around forever.
Here are some ideas:

1. Disable preemption, process a small number of small packets
   directly in an atomic context. This should cut latency
   down significantly, the tricky part is to only do it
   on a light load and disable this
   for the streaming case otherwise it's unfair.
   This might fail, if it does just bounce things out to
   a thread.

2. Switch to unsafe_put_user/unsafe_get_user,
   and batch up multiple accesses.

3. Allow adding a fixup point manually,
   such that multiple independent get_user accesses
   can get a single fixup (will allow better compiler
   optimizations).

> Jason Wang (3):
>   vhost: generalize adding used elem
>   vhost: fine grain userspace memory accessors
>   vhost: access vq metadata through kernel virtual address
> 
>  drivers/vhost/vhost.c | 281 ++++++++++++++++++++++++++++++++++++++----
>  drivers/vhost/vhost.h |  11 ++
>  2 files changed, 266 insertions(+), 26 deletions(-)
> 
> -- 
> 2.17.1