Re: [PATCH RFC] xen: if on Xen, "flatten" the scheduling domain hierarchy

[Juergen GroÃ]

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.

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. 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.

>   - whether you think, looking at this preliminary set of numbers, that
>     this is something worth continuing investigating,

I believe as both approaches lead to the same topology information used
by the scheduler (all vcpus are regarded as being equal) your numbers
should apply to my patch as well. Would you mind verifying this?

I still believe making the guest scheduler's decisions independant from
cpuid values is the way to go, as this will enable us to support more
scenarios (e.g. cpuid based licensing). For HVM guests and old PV guests
mangling the cpuid should still be done, though.

>   - if yes, what other workloads and benchmark it would make sense to
>     throw at it.

As you already mentioned an overcommitted host should be looked at as
well.


Thanks for doing the measurements,


Juergen
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