Re: WARNING in __mmdrop

[Jason Wang]

On 2019/7/25 äå9:26, 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