Re: [PATCH v3] alloc_tag: add per-NUMA node stats

[Kent Overstreet <kent.overstreet@linux.dev>]

On Thu, Jul 10, 2025 at 06:07:13PM -0700, Casey Chen wrote:
> On Thu, Jul 10, 2025 at 5:54 PM Kent Overstreet
> <kent.overstreet@linux.dev> wrote:
> >
> > On Thu, Jul 10, 2025 at 05:42:05PM -0700, Casey Chen wrote:
> > > Hi All,
> > >
> > > Thanks for reviewing my previous patches. I am replying some comments
> > > in our previous discussion
> > > https://lore.kernel.org/all/CAJuCfpHhSUhxer-6MP3503w6520YLfgBTGp7Q9Qm9kgN4TNsfw@mail.gmail.com/T/#u
> > >
> > > Most people care about the motivations and usages of this feature.
> > > Internally, we used to have systems having asymmetric memory to NUMA
> > > nodes. Node 0 uses a lot of memory but node 1 is pretty empty.
> > > Requests to allocate memory on node 0 always fail. With this patch, we
> > > can find the imbalance and optimize the memory usage. Also, David
> > > Rientjes and Sourav Panda provide their scenarios in which this patch
> > > would be very useful. It is easy to turn on an off so I think it is
> > > nice to have, enabling more scenarios in the future.
> > >
> > > Andrew / Kent,
> > > * I agree with Kent on using for_each_possible_cpu rather than
> > > for_each_online_cpu, considering CPU online/offline.
> > > * When failing to allocate counters for in-kernel alloc_tag, panic()
> > > is better than WARN(), eventually the kernel would panic at invalid
> > > memory access.
> > > * percpu stats would bloat data structures quite a bit.
> > >
> > > David Wang,
> > > I don't really understand what is 'granularity of calling sites'. If
> > > NUMA imbalance is found, the calling site could request memory
> > > allocation from different nodes. Other factors can affect NUMA
> > > balance, those information can be implemented in a different patch.
> >
> > Let's get this functionality in.
> >
> > We've already got userspace parsing and consuming /proc/allocinfo, so we
> > just need to do it without changing that format.
> 
> You mean keep the format without per-NUMA info the same as before ?
> My patch v3 changed the header and the alignment of bytes and calls. I
> can restore them back.

I mean an ioctl interface - so we can have a userspace program with
different switches for getting different types of output.

Otherwise the existing programs people have already written for
consuming /proc/allocinfo are going to break.