[External] Re: [RFC 1/2] riscv: process: Introduce idle thread using Zawrs extension

[Conor Dooley]

On Fri, Apr 19, 2024 at 12:14:47AM +0800, Xu Lu wrote:
> On Thu, Apr 18, 2024 at 11:06 PM Conor Dooley <conor at kernel.org> wrote:
> > On Thu, Apr 18, 2024 at 07:49:41PM +0800, Xu Lu wrote:

> > > +        The Zawrs extension defines a pair of instructions to be used
> > > +        in polling loops that allows a core to enter a low-power state
> > > +        and wait on a store to a memory location.
> > > +
> > > +        If you don't know what to do here, say Y.
> > > +
> > >  config TOOLCHAIN_HAS_V
> > >       bool
> > >       default y
> > > @@ -1075,6 +1090,15 @@ endmenu # "Power management options"
> > >
> > >  menu "CPU Power Management"
> > >
> > > +config RISCV_ZAWRS_IDLE
> > > +     bool "Idle thread using ZAWRS extensions"
> > > +     depends on RISCV_ISA_ZAWRS
> > > +     default y
> > > +     help
> > > +             Adds support to implement idle thread using ZAWRS extension.
> > > +
> > > +             If you don't know what to do here, say Y.
> >
> > I don't think this second option is needed, why would we not always want
> > to use the Zawrs version of this when it is available? Can we do it
> > unconditionally when RISCV_ISA_ZAWRS is set and the extension is
> > detected at runtime?
> >
> > Cheers,
> > Conor.
> 
> Indeed, we can always choose WRS.NTO when entering idle.
> 
> This config is introduced for the second commit in this patch series.
> In the second commit, we detect whether the target cpu is idle when
> sending IPI and write IPI info to the reserve set of idle cpu so as to
> avoid sending a physical IPI. Besides, the target idle cpu need not to
> go through traditional interrupt handling routine. However, if all
> cpus are busy and hardly enter idle, this commit may introduce
> performance overhead of extra instructions when sending IPI. Thus we
> introduce this config just in case.

Could you add the downsides into the help text of the config option so
that people can understand why to enable/disable the option?

Thanks,
Conor.
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