Re: [PATCH] RISC-V: Do not issue remote fences until smp is available

[Atish Patra]

On Thu, Nov 10, 2022 at 1:42 PM Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:
>
> On Mon, 31 Oct 2022 12:26:05 PDT (-0700), atishp@xxxxxxxxxxxxxx wrote:
> > On Mon, Oct 31, 2022 at 12:12 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >>
> >> On Fri, Oct 28, 2022 at 04:19:29PM -0700, Atish Patra wrote:
> >> > It is useless to issue remote fences if there is a single core
> >> > available. It becomes a bottleneck for sbi based rfences where
> >> > we will be making those ECALLs for no reason. Early code patching
> >> > because of static calls end up in this path.
> >> >
> >> > Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> >>
> >> Hey Atish,
> >> This doesn't apply for me to either fixes or for-next. What branch does
> >> it apply to?
> >> Thanks,
> >> Conor.
> >>
> >> > ---
> >> >  arch/riscv/mm/cacheflush.c | 4 ++++
> >> >  1 file changed, 4 insertions(+)
> >> >
> >> > diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> >> > index f10cb47eac3a..7fafc8c26505 100644
> >> > --- a/arch/riscv/mm/cacheflush.c
> >> > +++ b/arch/riscv/mm/cacheflush.c
> >> > @@ -19,6 +19,10 @@ void flush_icache_all(void)
> >> >  {
> >> >       local_flush_icache_all();
> >> >
> >> > +     /* No need to issue remote fence if only 1 cpu is online */
> >> > +     if (num_online_cpus() == 1)
> >> > +             return;
> >> > +
> >> >       if (IS_ENABLED(CONFIG_RISCV_SBI) && !riscv_use_ipi_for_rfence())
> >> >               sbi_remote_fence_i(NULL);
> >> >       else
> >> > --
> >> > 2.34.1
> >> >
> >
> > Sorry I forgot to specify the dependencies for this patch. This patch
> > is based on Anup's IPI series [1] as
> > I assumed the IPI series would go first. I can rebase on top of the
> > master if required.
> > However, the issue will manifest only after Jisheng's patch[2] which
> > moved the sbi_init to earlier and introduced the
> > static key in the paging_init path.
> >
> > [1] https://patchwork.kernel.org/project/linux-riscv/patch/20220820065446.389788-8-apatel@xxxxxxxxxxxxxxxx/
> > [2] https://lore.kernel.org/lkml/20220716115059.3509-1-jszhang@xxxxxxxxxx/
>
> IMO we should just stop issuing the SBI remote fences at all, with the
> code to do IPI-based fences we're just adding complexity for the slow
> case.

Sure. We can do that too. However, that will have some performance
impact for any platform(existing and future ones) without imsic.

Is that acceptable ? Maybe it will encourage every vendor to implement
AIA instead of PLIC ;)

--
Regards,
Atish