RE: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data accesses.

[David Laight]

From: Ingo Molnar
> Sent: 30 December 2023 11:09
> 
> 
> * Waiman Long <longman@xxxxxxxxxx> wrote:
> 
> > On 12/29/23 15:57, David Laight wrote:
> > > this_cpu_ptr() is rather more expensive than raw_cpu_read() since
> > > the latter can use an 'offset from register' (%gs for x86-84).
> > >
> > > Add a 'self' field to 'struct optimistic_spin_node' that can be
> > > read with raw_cpu_read(), initialise on first call.
> > >
> > > Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
> > > ---
> > >   kernel/locking/osq_lock.c | 14 +++++++++-----
> > >   1 file changed, 9 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > > index 9bb3a077ba92..b60b0add0161 100644
> > > --- a/kernel/locking/osq_lock.c
> > > +++ b/kernel/locking/osq_lock.c
> > > @@ -13,7 +13,7 @@
> > >    */
> > >   struct optimistic_spin_node {
> > > -	struct optimistic_spin_node *next, *prev;
> > > +	struct optimistic_spin_node *self, *next, *prev;
> > >   	int locked; /* 1 if lock acquired */
> > >   	int cpu; /* encoded CPU # + 1 value */
> > >   };
> > > @@ -93,12 +93,16 @@ osq_wait_next(struct optimistic_spin_queue *lock,
> > >   bool osq_lock(struct optimistic_spin_queue *lock)
> > >   {
> > > -	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> > > +	struct optimistic_spin_node *node = raw_cpu_read(osq_node.self);
> >
> > My gcc 11 compiler produces the following x86-64 code:
> >
> > 92        struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> >    0x0000000000000029 <+25>:    mov    %rcx,%rdx
> >    0x000000000000002c <+28>:    add %gs:0x0(%rip),%rdx        # 0x34
> > <osq_lock+36>
> >
> > Which looks pretty optimized for me. Maybe older compiler may generate more
> > complex code. However, I do have some doubt as to the benefit of this patch
> > at the expense of making the code a bit more complex.

My changed code is one instruction shorter!
  18:   65 48 8b 15 00 00 00    mov    %gs:0x0(%rip),%rdx        # 20 <osq_lock+0x20>
  1f:   00
                        1c: R_X86_64_PC32       .data..percpu..shared_aligned-0x4
However is might have one less cache line miss.

> GCC-11 is plenty of a look-back window in terms of compiler efficiency:
> latest enterprise distros use GCC-11 or newer, while recent desktop
> distros use GCC-13. Anything older won't matter, because no major
> distribution is going to use new kernels with old compilers.

There must be a difference in the header files as well.
Possibly forced by the older compiler I'm using (7.5 from Ubuntu 18.04).
But maybe based on some config option.

I'm seeing this_cpu_ptr(&xxx) converted to per_cpu_ptr(&xxx, smp_processor_id())
which necessitates an array lookup (indexed by cpu number).
Whereas I think you are seeing it implemented as
  raw_cpu_read(per_cpu_data_base) + offset_to(xxx)

So the old code generates (after the prologue):
  10:   49 89 fd                mov    %rdi,%r13
  13:   49 c7 c4 00 00 00 00    mov    $0x0,%r12
                        16: R_X86_64_32S        .data..percpu..shared_aligned
  1a:   e8 00 00 00 00          callq  1f <osq_lock+0x1f>
                        1b: R_X86_64_PC32       debug_smp_processor_id-0x4
  1f:   89 c0                   mov    %eax,%eax
  21:   48 8b 1c c5 00 00 00    mov    0x0(,%rax,8),%rbx
  28:   00
                        25: R_X86_64_32S        __per_cpu_offset
  29:   e8 00 00 00 00          callq  2e <osq_lock+0x2e>
                        2a: R_X86_64_PC32       debug_smp_processor_id-0x4
  2e:   4c 01 e3                add    %r12,%rbx
  31:   83 c0 01                add    $0x1,%eax
  34:   c7 43 10 00 00 00 00    movl   $0x0,0x10(%rbx)
  3b:   48 c7 03 00 00 00 00    movq   $0x0,(%rbx)
  42:   89 43 14                mov    %eax,0x14(%rbx)
  45:   41 87 45 00             xchg   %eax,0x0(%r13)

I was also surprised that smp_processor_id() is a real function rather
than an offset from %gs.

	David

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