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

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Message-ID: <CAOnJCUK5Z6i39f5MJaRFhorGjcmBR=p_MapY_TDcR1e274wtpA@mail.gmail.com>
Date:   Mon, 31 Oct 2022 12:26:05 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Conor Dooley <conor@...nel.org>
Cc:     Atish Patra <atishp@...osinc.com>, linux-kernel@...r.kernel.org,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Damien Le Moal <damien.lemoal@....com>,
        devicetree@...r.kernel.org, Jisheng Zhang <jszhang@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH] RISC-V: Do not issue remote fences until smp is available

On Mon, Oct 31, 2022 at 12:12 PM Conor Dooley <conor@...nel.org> 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@...osinc.com>
>
> 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@ventanamicro.com/
[2] https://lore.kernel.org/lkml/20220716115059.3509-1-jszhang@kernel.org/


--
Regards,
Atish