Messages in this thread | | | Subject | Re: Re: [Xen-devel] [PATCH RFC] xen: if on Xen, "flatten" the scheduling domain hierarchy | From | Juergen Gross <> | Date | Tue, 22 Sep 2015 06:42:51 +0200 |
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On 09/21/2015 07:49 AM, Juergen Gross wrote: > On 09/15/2015 06:50 PM, Dario Faggioli wrote: >> On Thu, 2015-08-20 at 20:16 +0200, Juergen Groß wrote: >>> On 08/18/2015 05:55 PM, Dario Faggioli wrote: >>>> Hey everyone, >>>> >>>> So, as a followup of what we were discussing in this thread: >>>> >>>> [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest >>>> >>>> http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html >>>> >>>> >>>> I started looking in more details at scheduling domains in the Linux >>>> kernel. Now, that thread was about CPUID and vNUMA, and their weird way >>>> of interacting, while this thing I'm proposing here is completely >>>> independent from them both. >>>> >>>> In fact, no matter whether vNUMA is supported and enabled, and no >>>> matter >>>> whether CPUID is reporting accurate, random, meaningful or completely >>>> misleading information, I think that we should do something about how >>>> scheduling domains are build. >>>> >>>> Fact is, unless we use 1:1, and immutable (across all the guest >>>> lifetime) pinning, scheduling domains should not be constructed, in >>>> Linux, by looking at *any* topology information, because that just does >>>> not make any sense, when vcpus move around. >>>> >>>> Let me state this again (hoping to make myself as clear as >>>> possible): no >>>> matter in how much good shape we put CPUID support, no matter how >>>> beautifully and consistently that will interact with both vNUMA, >>>> licensing requirements and whatever else. It will be always possible >>>> for >>>> vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and >>>> on two different NUMA nodes at time t2. Hence, the Linux scheduler >>>> should really not skew his load balancing logic toward any of those two >>>> situations, as neither of them could be considered correct (since >>>> nothing is!). >>>> >>>> For now, this only covers the PV case. HVM case shouldn't be any >>>> different, but I haven't looked at how to make the same thing happen in >>>> there as well. >>>> >>>> OVERALL DESCRIPTION >>>> =================== >>>> What this RFC patch does is, in the Xen PV case, configure scheduling >>>> domains in such a way that there is only one of them, spanning all the >>>> pCPUs of the guest. >>>> >>>> Note that the patch deals directly with scheduling domains, and >>>> there is >>>> no need to alter the masks that will then be used for building and >>>> reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). >>>> That is >>>> the main difference between it and the patch proposed by Juergen here: >>>> http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html >>>> >>>> >>>> This means that when, in future, we will fix CPUID handling and make it >>>> comply with whatever logic or requirements we want, that won't have >>>> any >>>> unexpected side effects on scheduling domains. >>>> >>>> Information about how the scheduling domains are being constructed >>>> during boot are available in `dmesg', if the kernel is booted with the >>>> 'sched_debug' parameter. It is also possible to look >>>> at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat. >>>> >>>> With the patch applied, only one scheduling domain is created, called >>>> the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can >>>> tell that from the fact that every cpu* folder >>>> in /proc/sys/kernel/sched_domain/ only have one subdirectory >>>> ('domain0'), with all the tweaks and the tunables for our scheduling >>>> domain. >>>> >>>> EVALUATION >>>> ========== >>>> I've tested this with UnixBench, and by looking at Xen build time, on a >>>> 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for >>>> now, but I plan to re-run them in DomUs soon (Juergen may be doing >>>> something similar to this in DomU already, AFAUI). >>>> >>>> I've run the benchmarks with and without the patch applied ('patched' >>>> and 'vanilla', respectively, in the tables below), and with different >>>> number of build jobs (in case of the Xen build) or of parallel copy of >>>> the benchmarks (in the case of UnixBench). >>>> >>>> What I get from the numbers is that the patch almost always brings >>>> benefits, in some cases even huge ones. There are a couple of cases >>>> where we regress, but always only slightly so, especially if comparing >>>> that to the magnitude of some of the improvement that we get. >>>> >>>> Bear also in mind that these results are gathered from Dom0, and >>>> without >>>> any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If >>>> we move things in DomU and do overcommit at the Xen scheduler level, I >>>> am expecting even better results. >>>> >>> ... >>>> REQUEST FOR COMMENTS >>>> ==================== >>>> Basically, the kind of feedback I'd be really glad to hear is: >>>> - what you guys thing of the approach, >>> >>> Yesterday at the end of the developer meeting we (Andrew, Elena and >>> myself) discussed this topic again. >>> >> Hey, >> >> Sorry for replying so late, I've been on vacation from right after >> XenSummit up until yesterday. :-) >> >>> Regarding a possible future scenario with credit2 eventually supporting >>> gang scheduling on hyperthreads (which is desirable due to security >>> reasons [side channel attack] and fairness) my patch seems to be more >>> suited for that direction than yours. >>> >> Ok. Just let me mention that 'Credit2 + gang scheduling' might not be >> exactly around the corner (although, we can prioritize working on it if >> we want). >> >> In principle, I think it's a really nice idea. I still don't have clear >> in mind how we would handle a couple of situations, but let's leave this >> aside for now, and stay on-topic. >> >>> Correct me if I'm wrong, but I >>> think scheduling domains won't enable the guest kernel's scheduler to >>> migrate threads more easily between hyperthreads opposed to other vcpus, >>> while my approach can easily be extended to do so. >>> >> I'm not sure I understand what you mean here. As far as the (Linux) >> scheduler is concerned, your patch and mine do the exact same thing: >> they arrange for the scheduling domains, when they're built, during >> boot, not to consider hyperthreads or multi-cores. >> >> Mine does it by removing the SMT (and the MC) level from the data >> structure in the scheduler that is used as a base for configuring the >> scheduling domains. Yours does it by making the topology bitmaps that >> are used at each one of those level all look the same. In fact, with >> your patch applied, I get the exact same situation as with mine, as far >> as scheduling domains are concerned: there is only one scheduling >> domain, with a different scheduling group for each vCPU inside it. > > Uuh, nearly. > > Your case won't deal correctly with NUMA, as the generic NUMA code is > using set_sched_topology() as well. One of NUMA and Xen will win and > overwrite the other's settings. > > To do things correctly you will have to handle NUMA as well.
One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.
Juergen
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