Re: [RFC PATCH v2 0/5] Reduce NUMA balance caused TLB-shootdowns in a VM

[Yan Zhao]

On Fri, Aug 11, 2023 at 12:35:27PM -0700, John Hubbard wrote:
> On 8/11/23 11:39, David Hildenbrand wrote:
> ...
> > > > Should we want to disable NUMA hinting for such VMAs instead (for example, by QEMU/hypervisor) that knows that any NUMA hinting activity on these ranges would be a complete waste of time? I recall that John H. once mentioned that there are
> > > similar issues with GPU memory:  NUMA hinting is actually counter-productive and they end up disabling it.
> > > > 
> > > 
> > > Yes, NUMA balancing is incredibly harmful to performance, for GPU and
> > > accelerators that map memory...and VMs as well, it seems. Basically,
> > > anything that has its own processors and page tables needs to be left
> > > strictly alone by NUMA balancing. Because the kernel is (still, even
> > > today) unaware of what those processors are doing, and so it has no way
> > > to do productive NUMA balancing.
> > 
> > Is there any existing way we could handle that better on a per-VMA level, or on the process level? Any magic toggles?
> > 
> > MMF_HAS_PINNED might be too restrictive. MMF_HAS_PINNED_LONGTERM might be better, but with things like iouring still too restrictive eventually.
> > 
> > I recall that setting a mempolicy could prevent auto-numa from getting active, but that might be undesired.
> > 
> > CCing Mel.
> > 
> 
> Let's discern between page pinning situations, and HMM-style situations.
> Page pinning of CPU memory is unnecessary when setting up for using that
> memory by modern GPUs or accelerators, because the latter can handle
> replayable page faults. So for such cases, the pages are in use by a GPU
> or accelerator, but unpinned.
> 
> The performance problem occurs because for those pages, the NUMA
> balancing causes unmapping, which generates callbacks to the device
> driver, which dutifully unmaps the pages from the GPU or accelerator,
> even if the GPU might be busy using those pages. The device promptly
> causes a device page fault, and the driver then re-establishes the
> device page table mapping, which is good until the next round of
> unmapping from the NUMA balancer.
> 
> hmm_range_fault()-based memory management in particular might benefit
> from having NUMA balancing disabled entirely for the memremap_pages()
> region, come to think of it. That seems relatively easy and clean at
> first glance anyway.
> 
> For other regions (allocated by the device driver), a per-VMA flag
> seems about right: VM_NO_NUMA_BALANCING ?
> 
Thanks a lot for those good suggestions!
For VMs, when could a per-VMA flag be set?
Might be hard in mmap() in QEMU because a VMA may not be used for DMA until
after it's mapped into VFIO.
Then, should VFIO set this flag on after it maps a range?
Could this flag be unset after device hot-unplug?