Re: [RFC PATCH v9 2/2] sched: Fix performance regression introduced by mm_cid

[Mathieu Desnoyers]

On 2023-04-20 08:50, Aaron Lu wrote:
> On Thu, Apr 20, 2023 at 08:41:05AM -0400, Mathieu Desnoyers wrote:
> > On 2023-04-20 05:56, Aaron Lu wrote:
> > > On Wed, Apr 19, 2023 at 11:50:12AM -0400, Mathieu Desnoyers wrote:
> > > > Introduce per-mm/cpu current concurrency id (mm_cid) to fix a PostgreSQL
> > > > sysbench regression reported by Aaron Lu.
> > >
> > > mm_cid_get() dropped to 5.x% after I disable CONFIG_DEBUG_PREEMPT, using
> > > __this_cpu_X() doesn't help, I suppose that is because __this_cpu_X()
> > > still needs to fetch mm->pcpu_cid.
> > >
> > > Annotate mm_cid_get():
> > >
> > >          │     static inline int mm_cid_get(struct mm_struct *mm)
> > >          │     {
> > >     0.05 │       push   %rbp
> > >     0.02 │       mov    %rsp,%rbp
> > >          │       push   %r15
> > >          │       push   %r14
> > >          │       push   %r13
> > >          │       push   %r12
> > >          │       push   %rbx
> > >     0.02 │       sub    $0x10,%rsp
> > >          │     struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid;
> > >    71.30 │       mov    0x60(%rdi),%r12
> > >          │     struct cpumask *cpumask;
> > >          │     int cid;
> > >          │
> > >          │     lockdep_assert_rq_held(rq);
> > >          │     cpumask = mm_cidmask(mm);
> > >          │     cid = __this_cpu_read(pcpu_cid->cid);
> > >    28.44 │       mov    %gs:0x8(%r12),%edx
> > >          │     if (mm_cid_is_valid(cid)) {
> > >
> > >
> > > sched_mm_cid_migrate_to() is 4.x% and its annotation :
> > >
> > >          │     dst_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(dst_rq));
> > >          │       mov     -0x30(%rbp),%rax
> > >    54.53 │       mov     0x60(%r13),%rbx
> > >    19.61 │       movslq  0xaf0(%rax),%r15
> > >
> > > The reason why accessing mm->pcpu_cid is so costly is still a myth to
> > > me...
> >
> > Then we clearly have another member of mm_struct on the same cache line as
> > pcpu_cid which is bouncing all over the place and causing false-sharing. Any
> > idea which field(s) are causing this ?
>
> That's my first reaction too but as I said in an earlier reply:
> https://lore.kernel.org/lkml/20230419080606.GA4247@ziqianlu-desk2/
> I've tried to place pcpu_cid into a dedicate cacheline with no other
> fields sharing a cacheline with it in mm_struct but it didn't help...

I see two possible culprits there:

1) The mm_struct pcpu_cid field is suffering from false-sharing. I would be
   interested to look at your attempt to move it to a separate cache line to
   try to figure out what is going on.

2) (Maybe?) The task_struct mm field is suffering from false-sharing and stalling
   the next instruction which needs to use its value to fetch the mm->pcpu_cid
   field. We could try moving the task_struct mm field into its own cache line to
   see if it helps.

Thanks,

Mathieu

> Thanks,
> Aaron

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com