Messages in this thread | | | Date | Fri, 15 Sep 2023 13:15:35 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] x86/percpu: Use raw_cpu_try_cmpxchg in preempt_count_set |
| |
* Ingo Molnar <mingo@kernel.org> wrote:
> > * Uros Bizjak <ubizjak@gmail.com> wrote: > > > Use raw_cpu_try_cmpxchg instead of raw_cpu_cmpxchg (*ptr, old, new) == old. > > x86 CMPXCHG instruction returns success in ZF flag, so this change saves a > > compare after cmpxchg (and related move instruction in front of cmpxchg). > > > > Also, raw_cpu_try_cmpxchg implicitly assigns old *ptr value to "old" when > > cmpxchg fails. There is no need to re-read the value in the loop. > > > > No functional change intended. > > > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Borislav Petkov <bp@alien8.de> > > Cc: Dave Hansen <dave.hansen@linux.intel.com> > > Cc: "H. Peter Anvin" <hpa@zytor.com> > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > > --- > > arch/x86/include/asm/preempt.h | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h > > index 2d13f25b1bd8..4527e1430c6d 100644 > > --- a/arch/x86/include/asm/preempt.h > > +++ b/arch/x86/include/asm/preempt.h > > @@ -31,11 +31,11 @@ static __always_inline void preempt_count_set(int pc) > > { > > int old, new; > > > > + old = raw_cpu_read_4(pcpu_hot.preempt_count); > > do { > > - old = raw_cpu_read_4(pcpu_hot.preempt_count); > > new = (old & PREEMPT_NEED_RESCHED) | > > (pc & ~PREEMPT_NEED_RESCHED); > > - } while (raw_cpu_cmpxchg_4(pcpu_hot.preempt_count, old, new) != old); > > + } while (!raw_cpu_try_cmpxchg_4(pcpu_hot.preempt_count, &old, new)); > > It would be really nice to have a before/after comparison of generated > assembly code in the changelog, to demonstrate the effectiveness of this > optimization.
Never mind, you did exactly that in the September 6 variation of these changes. I'll apply those.
Thanks,
Ingo
| |