Re: [PATCH] PCI: Warn about host bridge device when its numa node is NO_NODE

[Michal Hocko <mhocko@kernel.org>]

On Thu 24-10-19 12:40:13, Bjorn Helgaas wrote:
> On Thu, Oct 24, 2019 at 11:20:13AM +0200, Michal Hocko wrote:
> > On Wed 23-10-19 12:10:39, Bjorn Helgaas wrote:
> > > On Wed, Oct 23, 2019 at 04:22:43PM +0800, Yunsheng Lin wrote:
> > > > On 2019/10/23 5:04, Bjorn Helgaas wrote:
> > > > > On Sat, Oct 19, 2019 at 02:45:43PM +0800, Yunsheng Lin wrote:
> > > 
> > > > > I think the underlying problem you're addressing is that:
> > > > > 
> > > > >   - NUMA_NO_NODE == -1,
> > > > >   - dev_to_node(dev) may return NUMA_NO_NODE,
> > > > >   - kmalloc(dev) relies on cpumask_of_node(dev_to_node(dev)), and
> > > > >   - cpumask_of_node(NUMA_NO_NODE) makes an invalid array reference
> > > > > 
> > > > > For example, on arm64, mips loongson, s390, and x86,
> > > > > cpumask_of_node(node) returns "node_to_cpumask_map[node]", and -1 is
> > > > > an invalid array index.
> > > > 
> > > > The invalid array index of -1 is the underlying problem here when
> > > > cpumask_of_node(dev_to_node(dev)) is called and cpumask_of_node()
> > > > is not NUMA_NO_NODE aware yet.
> > > > 
> > > > In the "numa: make node_to_cpumask_map() NUMA_NO_NODE aware" thread
> > > > disscusion, it is requested that it is better to warn about the pcie
> > > > device without a node assigned by the firmware before making the
> > > > cpumask_of_node() NUMA_NO_NODE aware, so that the system with pci
> > > > devices of "NUMA_NO_NODE" node can be fixed by their vendor.
> > > > 
> > > > See: https://lore.kernel.org/lkml/20191011111539.GX2311@hirez.programming.kicks-ass.net/
> > > 
> > > Right.  We should warn if the NUMA node number would help us but DT or
> > > the firmware didn't give us one.
> > > 
> > > But we can do that independently of any cpumask_of_node() changes.
> > > There's no need to do one patch before the other.  Even if you make
> > > cpumask_of_node() tolerate NUMA_NO_NODE, we'll still get the warning
> > > because we're not actually changing any node assignments.
> > 
> > Agreed. And this has been proposed previously I believe but my
> > understanding was that Petr was against that.
> 
> I don't think Peter was opposed to a warning.

Now, he was opposed to cpumask_of_node handling IIRC.

> I assume you're
> referring to [1], which is about how cpumask_of_node() should work.
> That's not my area, so I don't have a strong opinion.  From that
> discussion:
> 
> Yunsheng> From what I can see, the problem can be fixed in three
> Yunsheng> place:
> Yunsheng> 1. Make user dev_to_node return a valid node id
> Yunsheng>    even when proximity domain is not set by bios(or node id
> Yunsheng>    set by buggy bios is not valid), which may need info from
> Yunsheng>    the numa system to make sure it will return a valid node.
> Yunsheng>
> Yunsheng> 2. User that call cpumask_of_node should ensure the node
> Yunsheng>    id is valid before calling cpumask_of_node, and user also
> Yunsheng>    need some info to make ensure node id is valid.
> Yunsheng>
> Yunsheng> 3. Make sure cpumask_of_node deal with invalid node id as
> Yunsheng>    this patchset.
> 
> Peter> 1) because even it is not set, the device really does belong
> Peter> to a node.  It is impossible a device will have magic
> Peter> uniform access to memory when CPUs cannot.
> Peter> 
> Peter> 2) is already true today, cpumask_of_node() requires a valid
> Peter> node_id.
> Peter> 
> Peter> 3) is just wrong and increases overhead for everyone.
> 
> I think Peter is advocating (1), i.e., if firmware doesn't tell us a
> node ID, we just pick one.  We can certainly do that in *addition* to
> adding a warning.  I'd like it to be a separate patch because I think
> we want the warning no matter what so users have some clue that
> performance could be better.

Yes, those should be two different patches.

> If we pick one, I guess we either assign some default, like node 0, to
> everything; or we somehow pick a random node to assign.

I have tried to explain that picking a default node is tricky because
node 0 is not generally available and you never know when a node might
just disappear if the device is not bound to it.

I really do not see why the proposed online_cpu_mask for that case is
such a big deal. It will likely lead to suboptimal performance but what
do you expect from a suboptimal HW description. There is no question
that the proper node affinity should be set and the warning might really
help here but trying to be clever and find a replacement sounds like
potential for more subtly broken results than doing a straightforward
thing.

But I will just go silent now because I am just repeating myself.

-- 
Michal Hocko
SUSE Labs