On Fri, Jul 26, 2019 at 09:36:18PM +0800, Jason Wang wrote:

On 2019/7/26 下午8:53, Jason Wang wrote:

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On 2019/7/26 下午8:38, Michael S. Tsirkin wrote:

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On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote:

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

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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:

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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.

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

Ok, let me retry if necessary (but I do remember I end up with deadlocks
last try).

Ok, I play a little with this. And it works so far. Will do more testing
tomorrow.

One reason could be I switch to use get_user_pages_fast() to
__get_user_pages_fast() which doesn't need mmap_sem.

Thanks

OK that sounds good. If we also set a flag to make
vhost_exceeds_weight exit, then I think it will be all good.

-- 
MST

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