Messages in this thread | | | Subject | Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker | From | Jason Wang <> | Date | Mon, 5 Aug 2019 16:24:27 +0800 |
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On 2019/8/5 下午2:40, Michael S. Tsirkin wrote: > On Mon, Aug 05, 2019 at 12:41:45PM +0800, Jason Wang wrote: >> On 2019/8/5 下午12:36, Jason Wang wrote: >>> On 2019/8/2 下午10:27, Michael S. Tsirkin wrote: >>>> On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote: >>>>> On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: >>>>>>> This must be a proper barrier, like a spinlock, mutex, or >>>>>>> synchronize_rcu. >>>>>> I start with synchronize_rcu() but both you and Michael raise some >>>>>> concern. >>>>> I've also idly wondered if calling synchronize_rcu() under the various >>>>> mm locks is a deadlock situation. >>>>> >>>>>> Then I try spinlock and mutex: >>>>>> >>>>>> 1) spinlock: add lots of overhead on datapath, this leads 0 >>>>>> performance >>>>>> improvement. >>>>> I think the topic here is correctness not performance improvement >>>> The topic is whether we should revert >>>> commit 7f466032dc9 ("vhost: access vq metadata through kernel >>>> virtual address") >>>> >>>> or keep it in. The only reason to keep it is performance. >>> >>> Maybe it's time to introduce the config option? >> >> Or does it make sense if I post a V3 with: >> >> - introduce config option and disable the optimization by default >> >> - switch from synchronize_rcu() to vhost_flush_work(), but the rest are the >> same >> >> This can give us some breath to decide which way should go for next release? >> >> Thanks > As is, with preempt enabled? Nope I don't think blocking an invalidator > on swap IO is ok, so I don't believe this stuff is going into this > release at this point. > > So it's more a question of whether it's better to revert and apply a clean > patch on top, or just keep the code around but disabled with an ifdef as is. > I'm open to both options, and would like your opinion on this.
Then I prefer to leave current code (VHOST_ARCH_CAN_ACCEL to 0) as is. This can also save efforts on rebasing packed virtqueues.
Thanks
> >>> >>>> Now as long as all this code is disabled anyway, we can experiment a >>>> bit. >>>> >>>> I personally feel we would be best served by having two code paths: >>>> >>>> - Access to VM memory directly mapped into kernel >>>> - Access to userspace >>>> >>>> >>>> Having it all cleanly split will allow a bunch of optimizations, for >>>> example for years now we planned to be able to process an incoming short >>>> packet directly on softirq path, or an outgoing on directly within >>>> eventfd. >>> >>> It's not hard consider we've already had our own accssors. But the >>> question is (as asked in another thread), do you want permanent GUP or >>> still use MMU notifiers. >>> >>> Thanks >>> >>> _______________________________________________ >>> Virtualization mailing list >>> Virtualization@lists.linux-foundation.org >>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
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