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SubjectRe: [PATCH -v5][RFC]: mutex: implement adaptive spinning

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 7 Jan 2009 22:32:22 +0100
> Ingo Molnar <mingo@elte.hu> wrote:
>
> > > We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); ..
> > > set_fs(oldfs);" crud, but it would probably be better to just add an
> > > architected accessor. Especially since it's going to generally just be a
> > >
> > > #define get_kernel_careful(val,p) __get_user(val,p)
> > >
> > > for most architectures.
> > >
> > > We've needed that before (and yes, we've simply mis-used __get_user() on
> > > x86 before rather than add it).
> >
> > for the oldfs stuff we already have probe_kernel_read(). OTOH, that
> > involves pagefault_disable() which is an atomic op
>
> tisn't. pagefault_disable() is just preempt_count()+=1;barrier() ?

okay. Not an atomic (which is plenty fast on Nehalem with 20 cycles
anyway), but probe_kernel_read() is expensive nevertheless:

ffffffff8027c092 <probe_kernel_read>:
ffffffff8027c092: 65 48 8b 04 25 10 00 mov %gs:0x10,%rax
ffffffff8027c099: 00 00
ffffffff8027c09b: 53 push %rbx
ffffffff8027c09c: 48 8b 98 48 e0 ff ff mov -0x1fb8(%rax),%rbx
ffffffff8027c0a3: 48 c7 80 48 e0 ff ff movq $0xffffffffffffffff,-0x1fb8(%rax)
ffffffff8027c0aa: ff ff ff ff
ffffffff8027c0ae: 65 48 8b 04 25 10 00 mov %gs:0x10,%rax
ffffffff8027c0b5: 00 00
ffffffff8027c0b7: ff 80 44 e0 ff ff incl -0x1fbc(%rax)
ffffffff8027c0bd: e8 0e dd 0d 00 callq ffffffff80359dd0 <__copy_from_user_inatomic>
ffffffff8027c0c2: 65 48 8b 14 25 10 00 mov %gs:0x10,%rdx
ffffffff8027c0c9: 00 00
ffffffff8027c0cb: ff 8a 44 e0 ff ff decl -0x1fbc(%rdx)
ffffffff8027c0d1: 65 48 8b 14 25 10 00 mov %gs:0x10,%rdx
ffffffff8027c0d8: 00 00
ffffffff8027c0da: 48 83 f8 01 cmp $0x1,%rax
ffffffff8027c0de: 48 89 9a 48 e0 ff ff mov %rbx,-0x1fb8(%rdx)
ffffffff8027c0e5: 48 19 c0 sbb %rax,%rax
ffffffff8027c0e8: 48 f7 d0 not %rax
ffffffff8027c0eb: 48 83 e0 f2 and $0xfffffffffffffff2,%rax
ffffffff8027c0ef: 5b pop %rbx
ffffffff8027c0f0: c3 retq
ffffffff8027c0f1: 90 nop

where __copy_user_inatomic() goes into the full __copy_generic_unrolled().
Not pretty.

> Am suspecting that you guys might be over-optimising this
> contended-path-were-going-to-spin-anyway code?

not sure. Especially for 'good' locking usage - where there are shortly
held locks and the spin times are short, the average time to get _out_ of
the spinning section is a kind of secondary fastpath as well.

Ingo


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