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

[Aaron Lu]

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...

Thanks,
Aaron