Messages in this thread Patch in this message | | | From | Uros Bizjak <> | Date | Wed, 18 Oct 2023 21:33:18 +0200 | Subject | Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr() |
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On Wed, Oct 18, 2023 at 8:26 PM Uros Bizjak <ubizjak@gmail.com> wrote: > > On Wed, Oct 18, 2023 at 8:16 PM Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > On Wed, 18 Oct 2023 at 11:08, Uros Bizjak <ubizjak@gmail.com> wrote: > > > > > > But loads from non-const memory work like the above. > > > > Yes, I'm certainly ok with the move to use plain loads from __seg_gs > > for the percpu accesses. If they didn't honor the memory clobber, we > > could never use it at all. > > > > I was just saying that the 'const' alias trick isn't useful for > > anything else than 'current', because everything else needs to at > > least honor our existing barriers. > > FYI, smp_processor_id() is implemented as: > > #define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number) > > where __this_* forces volatile access which disables CSE. > > *If* the variable is really stable, then it should use __raw_cpu_read. > Both, __raw_* and __this_* were recently (tip/percpu branch) > implemented for SEG_SUPPORT as:
This pach works for me:
--cut here-- diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h index 4fab2ed454f3..6eda4748bf64 100644 --- a/arch/x86/include/asm/smp.h +++ b/arch/x86/include/asm/smp.h @@ -141,8 +141,7 @@ __visible void smp_call_function_single_interrupt(struct pt_regs *r); * This function is needed by all SMP systems. It must _always_ be valid * from the initial startup. */ -#define raw_smp_processor_id() this_cpu_read(pcpu_hot.cpu_number) -#define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number) +#define raw_smp_processor_id() raw_cpu_read(pcpu_hot.cpu_number)
#ifdef CONFIG_X86_32 extern int safe_smp_processor_id(void); --cut here--
But removes merely 10 reads from 3219. BTW: I also don't understand the comment from include/linux/smp.h:
/* * Allow the architecture to differentiate between a stable and unstable read. * For example, x86 uses an IRQ-safe asm-volatile read for the unstable but a * regular asm read for the stable. */ #ifndef __smp_processor_id #define __smp_processor_id(x) raw_smp_processor_id(x) #endif All reads up to word size on x86 are atomic, so IRQ safe. asm-volatile is not some IRQ property, but prevents the compiler from CSE the asm and scheduling (moving) asm around too much.
Uros.
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