On 2019/7/26 下午7:49, Michael S. Tsirkin wrote:

On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote:

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On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:

quoted

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Exactly, and that's the reason actually I use synchronize_rcu() there.

So the concern is still the possible synchronize_expedited()?

I think synchronize_srcu_expedited.

synchronize_expedited sends lots of IPI and is bad for realtime VMs.

quoted

Can I do this
on through another series on top of the incoming V2?

Thanks

The question is this: is this still a gain if we switch to the
more expensive srcu? If yes then we can keep the feature on,

I think we only care about the cost on srcu_read_lock() which looks pretty
tiny form my point of view. Which is basically a READ_ONCE() + WRITE_ONCE().

Of course I can benchmark to see the difference.

quoted

if not we'll put it off until next release and think
of better solutions. rcu->srcu is just a find and replace,
don't see why we need to defer that. can be a separate patch
for sure, but we need to know how well it works.

I think I get here, let me try to do that in V2 and let's see the numbers.

Thanks

It looks to me for tree rcu, its srcu_read_lock() have a mb() which is 
too expensive for us.

If we just worry about the IPI, can we do something like in 
vhost_invalidate_vq_start()?

         if (map) {
                 /* In order to avoid possible IPIs with
                  * synchronize_rcu_expedited() we use call_rcu() +
                  * completion.
*/
init_completion(&c.completion);
                 call_rcu(&c.rcu_head, vhost_finish_vq_invalidation);
wait_for_completion(&c.completion);
                 vhost_set_map_dirty(vq, map, index);
vhost_map_unprefetch(map);
         }

?

There's one other thing that bothers me, and that is that
for large rings which are not physically contiguous
we don't implement the optimization.

For sure, that can wait, but I think eventually we should
vmap large rings.

Yes, worth to try. But using direct map has its own advantage: it can 
use hugepage that vmap can't

Thanks


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