On 21/11/17 15:29, Jan Beulich wrote: >>>> On 21.11.17 at 15:07, <igor.druzhi...@citrix.com> wrote: >> On 21/11/17 13:22, Jan Beulich wrote: >>>>>> On 09.11.17 at 15:49, <jbeul...@suse.com> wrote: >>>> See the code comment being added for why we need this. >>>> >>>> Reported-by: Igor Druzhinin <igor.druzhi...@citrix.com> >>>> Signed-off-by: Jan Beulich <jbeul...@suse.com> >>> >>> I realize we aren't settled yet on where to put the sync call. The >>> discussion appears to have stalled, though. Just to recap, >>> alternatives to the placement below are >>> - at the top of complete_domain_destroy(), being the specific >>> RCU callback exhibiting the problem (others are unlikely to >>> touch guest state) >>> - in rcu_do_batch(), paralleling the similar call from >>> do_tasklet_work() >> >> rcu_do_batch() sounds better to me. As I said before I think that the >> problem is general for the hypervisor (not for VMX only) and might >> appear in other places as well. > > The question here is: In what other cases do we expect an RCU > callback to possibly touch guest state? I think the common use is > to merely free some memory in a delayed fashion. >
I don't know for sure what the common scenario is for Xen but drawing parallels between Linux - you're probably right. >> Those choices that you outlined appear to be different in terms whether >> we solve the general problem and probably have some minor performance >> impact or we solve the ad-hoc problem but make the system more >> entangled. Here I'm more inclined to the first choice because this >> particular scenario the performance impact should be negligible. > > For the problem at hand there's no question about a > performance effect. The question is whether doing this for _other_ > RCU callbacks would introduce a performance drop in certain cases. > Yes, right. In that case this placement would mean we are going to lose the partial context each time we take RCU in idle, is this correct? If so that sounds like a common scenario to me and means there will be some performance degradation, although I don't know how common it really is. Anyway, if you're in favor of the previous approach I have no objections as my understanding of Xen codebase is still partial. Igor _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
