From: Ingo Molnar
> Sent: 30 December 2023 11:09
>
>
> * Waiman Long <long...@redhat.com> 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.lai...@aculab.com>
> > > ---
> > > 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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