RISC-V: Do not issue remote fences until smp is available

  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@rivosinc.com>
---
 arch/riscv/mm/cacheflush.c | 4 ++++
 1 file changed, 4 insertions(+)
  

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@rivosinc.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
>
  

On Mon, Oct 31, 2022 at 12:12 PM Conor Dooley <conor@kernel.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@rivosinc.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
  

On Mon, 31 Oct 2022 12:26:05 PDT (-0700), atishp@atishpatra.org wrote:
> On Mon, Oct 31, 2022 at 12:12 PM Conor Dooley <conor@kernel.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@rivosinc.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/

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.
  

On Thu, Nov 10, 2022 at 1:42 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Mon, 31 Oct 2022 12:26:05 PDT (-0700), atishp@atishpatra.org wrote:
> > On Mon, Oct 31, 2022 at 12:12 PM Conor Dooley <conor@kernel.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@rivosinc.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/
>
> 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