On Thu, Jan 21, 2021 at 7:19 PM Vlastimil Babka [off-list ref] wrote:

On 1/21/21 11:01 AM, Christoph Lameter wrote:

quoted

On Thu, 21 Jan 2021, Bharata B Rao wrote:

quoted

quoted

The problem is that calculate_order() is called a number of times
before secondaries CPUs are booted and it returns 1 instead of 224.
This makes the use of num_online_cpus() irrelevant for those cases

After adding in my command line "slub_min_objects=36" which equals to
4 * (fls(num_online_cpus()) + 1) with a correct num_online_cpus == 224
, the regression diseapears:

9 iterations of hackbench -l 16000 -g 16: 3.201sec (+/- 0.90%)

I'm surprised that hackbench is that sensitive to slab performance, anyway. It's
supposed to be a scheduler benchmark? What exactly is going on?

Uuuh, I think powerpc doesn't have cmpxchg_double?

"vgrep cmpxchg_double arch/" just spits out arm64, s390 and x86? And
<https://liblfds.org/mediawiki/index.php?title=Article:CAS_and_LL/SC_Implementation_Details_by_Processor_family>
says under "POWERPC": "no DW LL/SC"

So powerpc is probably hitting the page-bitlock-based implementation
all the time for stuff like __slub_free()? Do you have detailed
profiling results from "perf top" or something like that?

(I actually have some WIP patches and a design document for getting
rid of cmpxchg_double in struct page that I hacked together in the
last couple days; I'm currently in the process of sending them over to
some other folks in the company who hopefully have cycles to
review/polish/benchmark them so that they can be upstreamed, assuming
that those folks think they're important enough. I don't have the
cycles for it...)