Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Fri, 26 Jul 2019 20:53:18 +0800 |
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On 2019/7/26 下午8:38, Michael S. Tsirkin wrote: > 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 ...
Ok, let me retry if necessary (but I do remember I end up with deadlocks last try).
> > >> If we just worry about the IPI, > With synchronize_rcu what I would worry about is that guest is stalled
Can this synchronize_rcu() be triggered by guest? If yes, there are several other MMU notifiers that can block. Is vhost something special here?
> because system is busy because of other guests. > With expedited it's the IPIs... >
The current synchronize_rcu() can force a expedited grace period:
void synchronize_rcu(void) { ... if (rcu_blocking_is_gp()) return; if (rcu_gp_is_expedited()) synchronize_rcu_expedited(); else wait_rcu_gp(call_rcu); } EXPORT_SYMBOL_GPL(synchronize_rcu);
>> 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?
No faster but no IPI.
> > >>> 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.
Yes, that's possible but should be done on top.
Thanks
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