Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Fri, 26 Jul 2019 22:00:20 +0800 |
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On 2019/7/26 下午9:47, Michael S. Tsirkin wrote: > On Fri, Jul 26, 2019 at 08:53:18PM +0800, Jason Wang wrote: >> 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? > Sorry, let me explain: guests (and tasks in general) > can trigger activity that will > make synchronize_rcu take a long time.
Yes, I get this.
> Thus blocking > an mmu notifier until synchronize_rcu finishes > is a bad idea.
The question is, MMU notifier are allowed to be blocked on invalidate_range_start() which could be much slower than synchronize_rcu() to finish.
Looking at amdgpu_mn_invalidate_range_start_gfx() which calls amdgpu_mn_invalidate_node() which did:
r = reservation_object_wait_timeout_rcu(bo->tbo.resv, true, false, MAX_SCHEDULE_TIMEOUT);
...
>>> 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); > > An admin can force rcu to finish faster, trading > interrupts for responsiveness.
Yes, so when set, all each synchronize_rcu() will go for synchronize_rcu_expedited().
> >>>> 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. >> > Sorry I still don't see the point. > synchronize_rcu doesn't normally do an IPI either. >
Not the case of when rcu_expedited is set. This can just 100% make sure there's 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 > Absolutely. Need to fix up the bugs first. >
Yes.
Thanks
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