Re: [PATCH 0/3] slab: support memoryless nodes with sheaves

["Harry Yoo (Oracle)" <harry@kernel.org>]

On Wed, Apr 08, 2026 at 02:04:54PM +0100, Jon Hunter wrote:
> Hi Vlastimil,

Hi Jon,

> On 11/03/2026 17:22, Vlastimil Babka (SUSE) wrote:
> > On 3/11/26 10:49, Ming Lei wrote:
> > > On Wed, Mar 11, 2026 at 09:25:54AM +0100, Vlastimil Babka (SUSE) wrote:
> > > > This is the draft patch from [1] turned into a proper series with
> > > > incremental changes. It's based on v7.0-rc3. It's too intrusive for a
> > > > 7.0 hotfix, so we'll only be able to fix/reduce the regression in 7.1. I
> > > > hope it's acceptable given it's a non-standard configuration, 7.0 is not
> > > > a LTS, and it's a perf regression, not functionality.
> > > > 
> > > > Ming can you please retest this on top of v7.0-rc3, which already has
> > > > fb1091febd66 ("mm/slab: allow sheaf refill if blocking is not
> > > > allowed"). Separate data point for v7.0-rc3 could be also useful.
> > > > 
> > > > [1] https://lore.kernel.org/all/c6a01f7e-c6eb-454b-9b9e-734526dd659d@kernel.org/
> > > > 
> > > > Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> > > > ---
> > > > Vlastimil Babka (SUSE) (3):
> > > >        slab: decouple pointer to barn from kmem_cache_node
> > > >        slab: create barns for online memoryless nodes
> > > >        slab: free remote objects to sheaves on memoryless nodes
> > > 
> > > Hi Vlastimil and Guys,
> > > 
> > > I re-run the test case used in https://lore.kernel.org/all/aZ0SbIqaIkwoW2mB@fedora/
> > > 
> > > - v6.19-rc5: 34M
> > > 
> > > - 815c8e35511d Merge branch 'slab/for-7.0/sheaves' into slab/for-next: 13M
> > > 
> > > - v7.0-rc3: 13M
> > 
> > Thanks, that's in line with your previous testing of "mm/slab: allow sheaf
> > refill if blocking is not allowed" making no difference here. At least we
> > just learned it helps other benchmarks :)
> > 
> > > - v7.0-rc3 + the three patches: 24M
> > 
> > OK. So now it might be really the total per-cpu caching capacity difference.
> 
> I have also observed a performance regresssion for Linux v7.0-rc for some
> graphics related tests we run. I bisected to ...
> 
> # first bad commit: [e47c897a29491ade20b27612fdd3107c39a07357] slab: add
> sheaves to most caches
> 
> I came across Ming's report and hence, found this series. I have also tested
> the 3 patches in this series and it did appear to help with one test, but
> overall I am still seeing a ~25% performance regression (the tests are
> taking about 25% longer to run). I am not the owner or author of these
> specific tests and I have not dived into see exactly what is taking longer,
> but I just know they are taking longer to run.
> 
> Anyway, I have not seen any recent updates on this, and so I am not sure if
> there are any other updates or what the current status of this is?

As far as I remember we didn't get to fully recovering the performance
yet. Interestingly even when most of allocations go through the fastpath
it didn't fully recover [1].

[1] https://lore.kernel.org/all/abI9DKxuwl_4Gasj@hyeyoo

I was suspecting it's probably because of:
  - false sharing on something (sheaves, obj metadata, etc.), or
  - suboptimal NUMA placement, or
  - something outside slab involved

But I don't have enough data to back up any of these theories yet.

> If there are any more patches available I will be happy to test.

Thanks!

Before diving deeper, could you please share the NUMA topology from
`numactl -H` on your machine?

It's probably a NUMA machine? (and hopefully not memoryless ones!)

-- 
Cheers,
Harry / Hyeonggon