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

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Message-ID: <CAJF2gTSw6YebWt09W3xsux+rNLeefCwYYhQ5F5jJCWcxRWtK_Q@mail.gmail.com>
Date: Fri, 5 Jan 2024 09:13:20 +0800
From: Guo Ren <guoren@...nel.org>
To: Leonardo Bras <leobras@...hat.com>
Cc: Andrew Jones <ajones@...tanamicro.com>, paul.walmsley@...ive.com, palmer@...belt.com, 
	panqinglin2020@...as.ac.cn, bjorn@...osinc.com, conor.dooley@...rochip.com, 
	peterz@...radead.org, keescook@...omium.org, wuwei2016@...as.ac.cn, 
	xiaoguang.xing@...hgo.com, chao.wei@...hgo.com, unicorn_wang@...look.com, 
	uwu@...nowy.me, jszhang@...nel.org, wefu@...hat.com, atishp@...shpatra.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V2 3/3] riscv: xchg: Prefetch the destination word for sc.w

On Thu, Jan 4, 2024 at 10:17 PM Leonardo Bras <leobras@...hat.com> wrote:
>
> On Thu, Jan 04, 2024 at 04:14:27PM +0800, Guo Ren wrote:
> > On Thu, Jan 4, 2024 at 11:56 AM Leonardo Bras <leobras@...hat.com> wrote:
> > >
> > > 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@...hat.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@...tanamicro.com> wrote:
> > > > > > >
> > > > > > > On Sun, Dec 31, 2023 at 03:29:53AM -0500, guoren@...nel.org wrote:
> > > > > > > > From: Guo Ren <guoren@...ux.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@...ux.alibaba.com>
> > > > > > > > Signed-off-by: Guo Ren <guoren@...nel.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
> > Yes, it has a comparison, so a store may not exist there.
> >
> > > exclusiveness for this cpu, where shared state would be enough.
> > > Is that correct?
> > Yes, exclusiveness would invalidate other harts' cache lines.
>
> I see.
>
> I recall a previous discussion on computer arch which stated that any LR
> would require to get a cacheline in exclusive state for lr/sc to work, but
> I went through the RISC-V lr/sc documentation and could not find any info
> about its cacheline behavior.

No, lr couldn't get a cacheline in exclusive, that would break the ISA design.
Think about "lr + wfe" pair.

>
> If this stands correct, the PREFETCH.W could be useful before every lr,
> right?
> (maybe that's the case for arm64 that you mentioned before)
The arm64 "lr + sc" cmpxchg version is not good, don't follow that.
They are moving to the LSE's cas instruction.


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


-- 
Best Regards
 Guo Ren