On 06/05/2019 13:58, Jan Beulich wrote: >>>> On 06.05.19 at 12:20, <jgr...@suse.com> wrote: >> On 06/05/2019 12:01, Jan Beulich wrote: >>>>>> On 06.05.19 at 11:23, <jgr...@suse.com> wrote: >>>> On 06/05/2019 10:57, Jan Beulich wrote: >>>>> . Yet then I'm a little puzzled by its use here in the first place. >>>>> Generally I think for_each_cpu() uses in __init functions are >>>>> problematic, as they then require further code elsewhere to >>>>> deal with hot-onlining. A pre-SMP-initcall plus use of CPU >>>>> notifiers is typically more appropriate. >>>> >>>> And that was mentioned in the cover letter: cpu hotplug is not yet >>>> handled (hence the RFC status of the series). >>>> >>>> When cpu hotplug is being added it might be appropriate to switch the >>>> scheme as you suggested. Right now the current solution is much more >>>> simple. >>> >>> I see (I did notice the cover letter remark, but managed to not >>> honor it when writing the reply), but I'm unconvinced if incurring >>> more code churn by not dealing with things the "dynamic" way >>> right away is indeed the "more simple" (overall) solution. >> >> Especially with hotplug things are becoming more complicated: I'd like >> to have the final version fall back to smaller granularities in case >> e.g. the user has selected socket scheduling and two sockets have >> different numbers of cores. With hotplug such a situation might be >> discovered only with some domUs already running, so how should we >> react in that case? Doing panic() is no option, so either we reject >> onlining the additional socket, or we adapt by dynamically modifying the >> scheduling granularity. Without that being discussed I don't think it >> makes sense to put a lot effort into a solution which is going to be >> rejected in the end. > > Hmm, where's the symmetry requirement coming from? Socket > scheduling should mean as many vCPU-s on one socket as there > are cores * threads; similarly core scheduling (number of threads). > Statically partitioning domains would seem an intermediate step > at best only anyway, as that requires (on average) leaving more > resources (cores/threads) idle than with a dynamic partitioning > model.
And that is exactly the purpose of core/socket scheduling. How else would it be possible (in future) to pass through the topology below the scheduling granularity to the guest? And how should it be of any use for fighting security issues due to side channel attacks? > As to your specific question how to react: Since bringing online > a full new socket implies bringing online all its cores / threads one > by one anyway, a "too small" socket in your scheme would > simply result in the socket remaining unused until "enough" > cores/threads have appeared. Similarly the socket would go > out of use as soon as one of its cores/threads gets offlined. Yes, this is a possible way to do it. It should be spelled out, though. > Obviously this ends up problematic for the last usable socket. Yes, like today for the last cpu/thread. > But with the static partitioning you describe I also can't really > see how "xen-hptool smt-disable" is going to work. It won't work. It just makes no sense to use it with core scheduling active. In the best case it would render all cores but the last one unusable (the last one would be refused to be disabled) and all domains would then fight for that last core. Juergen _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
