Subject: Re: [PATCH V3 3/3] vhost_net: basic polling support

From: Jason Wang <[email-masked]>
Date: Mon, 29 Feb 2016 13:15:48 +0800

On 02/28/2016 10:09 PM, Michael S. Tsirkin wrote:
> On Fri, Feb 26, 2016 at 04:42:44PM +0800, Jason Wang wrote:
>> > This patch tries to poll for new added tx buffer or socket receive
>> > queue for a while at the end of tx/rx processing. The maximum time
>> > spent on polling were specified through a new kind of vring ioctl.
>> > 
>> > Signed-off-by: Jason Wang <[email-masked]>
> Looks good overall, but I still see one problem.
>
>> > ---
>> >  drivers/vhost/net.c        | 79 +++++++++++++++++++++++++++++++++++++++++++---
>> >  drivers/vhost/vhost.c      | 14 ++++++++
>> >  drivers/vhost/vhost.h      |  1 +
>> >  include/uapi/linux/vhost.h |  6 ++++
>> >  4 files changed, 95 insertions(+), 5 deletions(-)
>> > 
>> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> > index 9eda69e..c91af93 100644
>> > --- a/drivers/vhost/net.c
>> > +++ b/drivers/vhost/net.c
>> > @@ -287,6 +287,44 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>> >  	rcu_read_unlock_bh();
>> >  }
>> >  
>> > +static inline unsigned long busy_clock(void)
>> > +{
>> > +	return local_clock() >> 10;
>> > +}
>> > +
>> > +static bool vhost_can_busy_poll(struct vhost_dev *dev,
>> > +				unsigned long endtime)
>> > +{
>> > +	return likely(!need_resched()) &&
>> > +	       likely(!time_after(busy_clock(), endtime)) &&
>> > +	       likely(!signal_pending(current)) &&
>> > +	       !vhost_has_work(dev) &&
>> > +	       single_task_running();
> So I find it quite unfortunate that this still uses single_task_running.
> This means that for example a SCHED_IDLE task will prevent polling from
> becoming active, and that seems like a bug, or at least
> an undocumented feature :).

Yes, it may need more thoughts.

>
> Unfortunately this logic affects the behaviour as observed
> by userspace, so we can't merge it like this and tune
> afterwards, since otherwise mangement tools will start
> depending on this logic.
>
>

How about remove single_task_running() first here and optimize on top?
We probably need something like this to handle overcommitment.