Re: [PATCH V2 3/3] riscv: xchg: Prefetch the destination word for sc.w

[Leonardo Bras <leobras@redhat.com>]

On Thu, Jan 04, 2024 at 09:24:40AM +0800, Guo Ren wrote:
> On Thu, Jan 4, 2024 at 3:45 AM Leonardo Bras <leobras@redhat.com> wrote:
> >
> > On Wed, Jan 03, 2024 at 02:15:45PM +0800, Guo Ren wrote:
> > > On Tue, Jan 2, 2024 at 7:19 PM Andrew Jones <ajones@ventanamicro.com> wrote:
> > > >
> > > > On Sun, Dec 31, 2023 at 03:29:53AM -0500, guoren@kernel.org wrote:
> > > > > From: Guo Ren <guoren@linux.alibaba.com>
> > > > >
> > > > > The cost of changing a cacheline from shared to exclusive state can be
> > > > > significant, especially when this is triggered by an exclusive store,
> > > > > since it may result in having to retry the transaction.
> > > > >
> > > > > This patch makes use of prefetch.w to prefetch cachelines for write
> > > > > prior to lr/sc loops when using the xchg_small atomic routine.
> > > > >
> > > > > This patch is inspired by commit: 0ea366f5e1b6 ("arm64: atomics:
> > > > > prefetch the destination word for write prior to stxr").
> > > > >
> > > > > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > > > > Signed-off-by: Guo Ren <guoren@kernel.org>
> > > > > ---
> > > > >  arch/riscv/include/asm/cmpxchg.h | 4 +++-
> > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > > > > index 26cea2395aae..d7b9d7951f08 100644
> > > > > --- a/arch/riscv/include/asm/cmpxchg.h
> > > > > +++ b/arch/riscv/include/asm/cmpxchg.h
> > > > > @@ -10,6 +10,7 @@
> > > > >
> > > > >  #include <asm/barrier.h>
> > > > >  #include <asm/fence.h>
> > > > > +#include <asm/processor.h>
> > > > >
> > > > >  #define __arch_xchg_masked(prepend, append, r, p, n)                 \
> > > >
> > > > Are you sure this is based on v6.7-rc7? Because I don't see this macro.
> > > Oh, it is based on Leobras' patches. I would remove it in the next of version.
> >
> > I would say this next :)
> Thx for the grammar correction.

Oh, I was not intending to correct grammar.
I just meant the next thing I would mention is that it was based on top of 
my patchset instead of v6.7-rc7:

> 
> >
> > >
> > > >
> > > > >  ({                                                                   \
> > > > > @@ -23,6 +24,7 @@
> > > > >                                                                       \
> > > > >       __asm__ __volatile__ (                                          \
> > > > >              prepend                                                  \
> > > > > +            PREFETCHW_ASM(%5)                                        \
> > > > >              "0:      lr.w %0, %2\n"                                  \
> > > > >              "        and  %1, %0, %z4\n"                             \
> > > > >              "        or   %1, %1, %z3\n"                             \
> > > > > @@ -30,7 +32,7 @@
> > > > >              "        bnez %1, 0b\n"                                  \
> > > > >              append                                                   \
> > > > >              : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b))       \
> > > > > -            : "rJ" (__newx), "rJ" (~__mask)                          \
> > > > > +            : "rJ" (__newx), "rJ" (~__mask), "rJ" (__ptr32b)         \
> > > >
> > > > I'm pretty sure we don't want to allow the J constraint for __ptr32b.
> > > >
> > > > >              : "memory");                                             \
> > > > >                                                                       \
> > > > >       r = (__typeof__(*(p)))((__retx & __mask) >> __s);               \
> > > > > --
> > > > > 2.40.1
> > > > >
> > > >
> > > > Thanks,
> > > > drew
> > >
> > >
> > >
> > > --
> > > Best Regards
> > >  Guo Ren
> > >
> >
> > Nice patch :)
> > Any reason it's not needed in __arch_cmpxchg_masked(), and __arch_cmpxchg() ?
> CAS is a conditional AMO, unlike xchg (Stand AMO). Arm64 is wrong, or
> they have a problem with the hardware.

Sorry, I was unable to fully understand the reason here.

You suggest that the PREFETCH.W was inserted on xchg_masked because it will 
always switch the variable (no compare, blind CAS), but not on cmpxchg.

Is this because cmpxchg will depend on a compare, and thus it does not 
garantee a write? so it would be unwise to always prefetch cacheline 
exclusiveness for this cpu, where shared state would be enough.
Is that correct? 

Thanks!
Leo


> 
> >
> > Thanks!
> > Leo
> >
> 
> 
> -- 
> Best Regards
>  Guo Ren
>