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Date:   Fri, 26 Jul 2019 08:38:13 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
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Subject: Re: WARNING in __mmdrop
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On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote:
> 
> 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:
> > > On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:
> > > > > 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.
> > > > 
> > > > > 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.
> > > 
> > > 
> > > > 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.

I will try to ponder using vq lock in some way.
Maybe with trylock somehow ...


> If we just worry about the IPI,

With synchronize_rcu what I would worry about is that guest is stalled
because system is busy because of other guests.
With expedited it's the IPIs...


> 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);
>         }
> 
> ?

Why would that be faster than synchronize_rcu?


> 
> > 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

Sure, so we can do that for small rings.

-- 
MST