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

From: Yan Zhao <yan.y.zhao@intel.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>, <kvm@vger.kernel.org>,
	<pbonzini@redhat.com>, <seanjc@google.com>,
	<mike.kravetz@oracle.com>, <apopple@nvidia.com>, <jgg@nvidia.com>,
	<rppt@kernel.org>, <akpm@linux-foundation.org>,
	<kevin.tian@intel.com>, Mel Gorman <mgorman@techsingularity.net>,
	<alex.williamson@redhat.com>
Subject: Re: [RFC PATCH v2 0/5] Reduce NUMA balance caused TLB-shootdowns in a VM
Date: Thu, 17 Aug 2023 13:05:07 +0800	[thread overview]
Message-ID: <ZN2qg4cPC2hEgtmY@yzhao56-desk.sh.intel.com> (raw)
In-Reply-To: <37325c27-223d-400d-bd86-34bdbfb92a5f@nvidia.com>

On Wed, Aug 16, 2023 at 11:00:36AM -0700, John Hubbard wrote:
> On 8/16/23 02:49, David Hildenbrand wrote:
> > But do 32bit architectures even care about NUMA hinting? If not, just
> > ignore them ...
> 
> Probably not!
> 
> ...
> > > So, do you mean that let kernel provide a per-VMA allow/disallow
> > > mechanism, and
> > > it's up to the user space to choose between per-VMA and complex way or
> > > global and simpler way?
> > 
> > QEMU could do either way. The question would be if a per-vma settings
> > makes sense for NUMA hinting.
> 
> From our experience with compute on GPUs, a per-mm setting would suffice.
> No need to go all the way to VMA granularity.
> 
After an offline internal discussion, we think a per-mm setting is also
enough for device passthrough in VMs.

BTW, if we want a per-VMA flag, compared to VM_NO_NUMA_BALANCING, do you
think it's of any value to providing a flag like VM_MAYDMA?
Auto NUMA balancing or other components can decide how to use it by
themselves.