Re: [Regression] mqueue performance degradation after "The new cgroup slab memory controller" patchset.

[Shakeel Butt <shakeelb@google.com>]

On Mon, Dec 5, 2022 at 8:31 AM Waiman Long <longman@redhat.com> wrote:
>
> On 12/5/22 11:06, Shakeel Butt wrote:
> > Hi Sven,
> >
> > On Mon, Dec 5, 2022 at 6:56 AM Luther, Sven <Sven.Luther@windriver.com> wrote:
> >> #regzbot ^introduced 10befea91b61c4e2c2d1df06a2e978d182fcf792
> >>
> >> We are making heavy use of mqueues, and noticed a degradation of performance between 4.18 & 5.10 linux kernels.
> >>
> >> After a gross per-version tracing, we did kernel bisection between 5.8 and 5.9
> >> and traced the issue to a 10 patches (of which 9 where skipped as they didn't boot) between:
> >>
> >>
> >> commit 10befea91b61c4e2c2d1df06a2e978d182fcf792 (HEAD, refs/bisect/bad)
> >> Author: Roman Gushchin <guro@fb.com>
> >> Date:   Thu Aug 6 23:21:27 2020 -0700
> >>
> >>      mm: memcg/slab: use a single set of kmem_caches for all allocations
> >>
> >> and:
> >>
> >> commit 286e04b8ed7a04279ae277f0f024430246ea5eec (refs/bisect/good-286e04b8ed7a04279ae277f0f024430246ea5eec)
> >> Author: Roman Gushchin <guro@fb.com>
> >> Date:   Thu Aug 6 23:20:52 2020 -0700
> >>
> >>      mm: memcg/slab: allocate obj_cgroups for non-root slab pages
> >>
> >> All of them are part of the "The new cgroup slab memory controller" patchset:
> >>
> >>    https://lore.kernel.org/all/20200623174037.3951353-18-guro@fb.com/T/
> >>
> >> from Roman Gushchin, which moves the accounting for page level to the object level.
> >>
> >> Measurements where done using the a test programmtest, which measures mix/average/max time mqueue_send/mqueue_rcv,
> >> and average for getppid, both measured over 100 000 runs. Results are shown in the following table
> >>
> >> +----------+--------------------------+-------------------------+----------------+
> >> | kernel   |    mqueue_rcv (ns)       | mqueue_send (ns)        |    getppid     |
> >> | version  | min avg  max   variation | min avg max   variation | (ns) variation |
> >> +----------+--------------------------+-------------------------+----------------+
> >> | 4.18.45  | 351 382 17533     base   | 383 410 13178     base  | 149      base  |
> >> | 5.8-good | 380 392  7156   -2,55%   | 376 384  6225    6,77%  | 169   -11,83%  |
> >> | 5.8-bad  | 524 530  5310  -27,92%   | 512 519  8775  -21,00%  | 169   -11,83%  |
> >> | 5.10     | 520 533  4078  -28,33%   | 518 534  8108  -23,22%  | 167   -10,78%  |
> >> | 5.15     | 431 444  8440  -13,96%   | 425 437  6170   -6,18%  | 171   -12,87%  |
> >> | 6.03     | 474 614  3881  -37,79%   | 482 693   931  -40,84%  | 171   -12,87%  |
> >> +----------+--------------------------+-------------------------+-----------------
> >>
> > Is the last kernel 6.0.3? Also we know there is performance impact of
> > per-object kmem accounting. Can you try the latest i.e. 6.1-rc8? There
> > are a couple of memcg charging optimization patches merged in this
> > window.
>
> It is known that per-object kmem accounting regresses performance. I had
> submitted a number of optimization patches that got merged into v5.14.
> So the regression is reduced in the 5.15 line above. It looks like there
> are some additional regressions in the latest kernel. We will need to
> figure out what causes it.
>

In 5.18 PREEMPT_RT patches went in and 559271146efc ("mm/memcg:
optimize user context object stock access") got reverted.