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

[Palmer Dabbelt]

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.