Re: [RFC][PATCH v2 00/21] PMEM NUMA node and hotness accounting/migration

[Fengguang Wu]

Hi Jonathan,

Thanks for showing the gap on tracking hot accesses from devices.

On Mon, Jan 28, 2019 at 05:42:39PM +0000, Jonathan Cameron wrote:
> On Wed, 2 Jan 2019 12:21:10 +0000
> Jonathan Cameron <jonathan.cameron@xxxxxxxxxx> wrote:
>
> > On Fri, 28 Dec 2018 20:52:24 +0100
> > Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > > [Ccing Mel and Andrea]
> > >
>
> Hi,
>
> I just wanted to highlight this section as I didn't feel we really addressed this
> in the earlier conversation.
>
> > * Hot pages may not be hot just because the host is using them a lot.  It would be
> >   very useful to have a means of adding information available from accelerators
> >   beyond simple accessed bits (dreaming ;)  One problem here is translation
> >   caches (ATCs) as they won't normally result in any updates to the page accessed
> >   bits.  The arm SMMU v3 spec for example makes it clear (though it's kind of
> >   obvious) that the ATS request is the only opportunity to update the accessed
> >   bit.  The nasty option here would be to periodically flush the ATC to force
> >   the access bit updates via repeats of the ATS request (ouch).
> >   That option only works if the iommu supports updating the accessed flag
> >   (optional on SMMU v3 for example).

If ATS based updates are supported, we may trigger it when closing the
/proc/pid/idle_pages file. We already do TLB flushes at that time. For
example,

[PATCH 15/21] ept-idle: EPT walk for virtual machine

       ept_idle_release():
         kvm_flush_remote_tlbs(kvm);

[PATCH 17/21] proc: introduce /proc/PID/idle_pages

       mm_idle_release():
         flush_tlb_mm(mm);

The flush cost is kind of "minimal necessary" in our current use
model, where user space scan+migration daemon will do such loop:

loop:
       walk page table N times:
               open,read,close /proc/PID/idle_pages
               (flushes TLB on file close)
               sleep for a short interval
       sort and migrate hot pages
       sleep for a while

> If we ignore the IOMMU hardware update issue which will simply need to be addressed
> by future hardware if these techniques become common, how do we address the
> Address Translation Cache issue without potentially causing big performance
> problems by flushing the cache just to force an accessed bit update?
>
> These devices are frequently used with PRI and Shared Virtual Addressing
> and can be accessing most of your memory without you having any visibility
> of it in the page tables (as they aren't walked if your ATC is well matched
> in size to your usecase.
>
> Classic example would be accelerated DB walkers like the the CCIX demo
> Xilinx has shown at a few conferences.   The whole point of those is that
> most of the time only your large set of database walkers is using your
> memory and they have translations cached for for a good part of what
> they are accessing.  Flushing that cache could hurt a lot.
> Pinning pages hurts for all the normal flexibility reasons.
>
> Last thing we want is to be migrating these pages that can be very hot but
> in an invisible fashion.

If there are some other way to get hotness for special device memory,
the user space daemon may be extended to cover that. Perhaps by
querying another new kernel interface.

By driving hotness accounting and migration in user space, we harvest
this kind of flexibility. In the daemon POV, /proc/PID/idle_pages
provides one common way to get "accessed" bits hence hotness, though
the daemon does not need to depend solely on it.

Thanks,
Fengguang