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

[Linus Torvalds]

On Sat, 30 Dec 2023 at 12:41, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> UNTESTED patch to just do the "this_cpu_write()" parts attached.
> Again, note how we do end up doing that this_cpu_ptr conversion later
> anyway, but at least it's off the critical path.

Also note that while 'this_cpu_ptr()' doesn't exactly generate lovely
code, it really is still better than caching a value in memory.

At least the memory location that 'this_cpu_ptr()' accesses is
slightly more likely to be hot (and is right next to the cpu number,
iirc).

That said, I think we should fix this_cpu_ptr() to not ever generate
that disgusting cltq just because the cpu pointer has the wrong
signedness. I don't quite know how to do it, but this:

  -#define per_cpu_offset(x) (__per_cpu_offset[x])
  +#define per_cpu_offset(x) (__per_cpu_offset[(unsigned)(x)])

at least helps a *bit*. It gets rid of the cltq, at least, but if
somebody actually passes in an 'unsigned long' cpuid, it would cause
an unnecessary truncation.

And gcc still generates

        subl    $1, %eax        #, cpu_nr
        addq    __per_cpu_offset(,%rax,8), %rcx

instead of just doing

        addq    __per_cpu_offset-8(,%rax,8), %rcx

because it still needs to clear the upper 32 bits and doesn't know
that the 'xchg()' already did that.

Oh well. I guess even without the -1/+1 games by the OSQ code, we
would still end up with a "movl" just to do that upper bits clearing
that the compiler doesn't know is unnecessary.

I don't think we have any reasonable way to tell the compiler that the
register output of our xchg() inline asm has the upper 32 bits clear.

              Linus