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

[Juergen Gross <jgross@xxxxxxxx>]

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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