Re: [PATCH UPDATED 1/3] vhost: replace vhost_workqueue with per-vhost kthread

Michael S. Tsirkin
2010-07-26T15:56:50Z

On Mon, Jul 26, 2010 at 05:34:44PM +0200, Tejun Heo wrote:
> Hello,
>
> On 07/26/2010 05:25 PM, Michael S. Tsirkin wrote:
> > BTW, kthread_worker would benefit from the optimization I implemented
> > here as well.
>
> Hmmm... I'm not quite sure whether it's an optimization. I thought
> the patch was due to feeling uncomfortable about using barriers?

Oh yes. But getting rid of barriers is what motivated me originally.

> Is it an optimization?
>
> Thanks.

Yes, sure. This removes atomic read and 2 barrier operations on data path. And
it does not add any new synchronization: instead, we reuse the lock that we
take anyway. The relevant part is:

+ if (work) {
+ __set_current_state(TASK_RUNNING);
+ work->fn(work);
+ } else
+ schedule();

- if (work) {
- __set_current_state(TASK_RUNNING);
- work->fn(work);
- smp_wmb(); /* wmb worker-b0 paired with flush-b1 */
- work->done_seq = work->queue_seq;
- smp_mb(); /* mb worker-b1 paired with flush-b0 */
- if (atomic_read(&work->flushing))
- wake_up_all(&work->done);
- } else
- schedule();
-
- goto repeat;

Is there a git tree with kthread_worker applied?
I might do this just for fun ...

> --
> tejun