[External] Re: [RFC 0/2] riscv: Idle thread using Zawrs extension

[Andrew Jones]

On Thu, Apr 18, 2024 at 09:09:06PM +0800, Xu Lu wrote:
> On Thu, Apr 18, 2024 at 8:56 PM Christoph Müllner
> <christoph.muellner at vrull.eu> wrote:
> >
> > On Thu, Apr 18, 2024 at 2:44 PM Xu Lu <luxu.kernel at bytedance.com> wrote:
> > >
> > > On Thu, Apr 18, 2024 at 8:26 PM Christoph Müllner
> > > <christoph.muellner at vrull.eu> wrote:
> > > >
> > > > On Thu, Apr 18, 2024 at 1:50 PM Xu Lu <luxu.kernel at bytedance.com> wrote:
> > > > >
> > > > > This patch series introduces a new implementation of idle thread using
> > > > > Zawrs extension.
> > > >
> > > > This overlaps with the following series:
> > > >   https://lore.kernel.org/all/20240315134009.580167-7-ajones@ventanamicro.com/
> > >
> > > Hi Christoph.
> > > Thanks for your reply!
> > > Actually our patch series is different from this. The work from your
> > > link focuses on providing support for Zawrs and implementing spinlock
> > > using it, while our work focuses on implementing idle thread using
> > > Zawrs and accelerating IPI to idle cpu. Of course, the ISA ZAWRS
> > > config part can be merged. We will refine our code in the next version
> > > to reduce code conflicts.
> >
> > Yes, I've seen that this targets another optimization, but the basic
> > Zawrs support
> > would be identical to the other patchset (even if it is not).
> > I would propose that we work on a basic Zawrs support patchset that introduces
> > the Kconfig, DTS and hwprobe parts (a subset of Andrew's patchset).
> > Once this is merged, all other optimizations can be built upon it
> > (spinlocks, idle thread, glibc CPU spinning).
> > If this proposal is fine for the maintainers/reviewers, then Andrew could resend
> > these basic-support patches.
> >
> > BR
> > Christoph
> 
> Roger that! This does make more sense. We will rebase our code on
> Andrew's basic support patches in the next version.

And I'm just about to send that next version. I'll send tomorrow morning
if not yet today.

Thanks,
drew



> 
> Regards,
> Xu Lu
> 
> >
> >
> > >
> > > >
> > > > BR
> > > > Christoph
> > > >
> > > > >
> > > > > The Zawrs[0] extension introduces two new instructions named WRS.STO and
> > > > > WRS.NTO in RISC-V. When software registers a reservation set using LR
> > > > > instruction, a subsequent WRS.STO or WRS.NTO instruction will cause the
> > > > > hart to stall in a low-power state until a store happens to the
> > > > > reservation set or an interrupt becomes pending. The difference between
> > > > > these two instructions is that WRS.STO will terminate stall after an
> > > > > implementation-defined timeout while WRS.NTO won't.
> > > > >
> > > > > This patch series implements idle thread using WRS.NTO instruction.
> > > > > Besides, we found there is no need to send a real IPI to wake up an idle
> > > > > CPU. Instead, we write IPI information to the reservation set of an idle
> > > > > CPU to wake it up and let it handle IPI quickly, without going through
> > > > > tranditional interrupt handling routine.
> > > > >
> > > > > [0] https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc
> > > > >
> > > > > Xu Lu (2):
> > > > >   riscv: process: Introduce idle thread using Zawrs extension
> > > > >   riscv: Use Zawrs to accelerate IPI to idle cpu
> > > > >
> > > > >  arch/riscv/Kconfig                 |  24 +++++++
> > > > >  arch/riscv/include/asm/cpuidle.h   |  11 +---
> > > > >  arch/riscv/include/asm/hwcap.h     |   1 +
> > > > >  arch/riscv/include/asm/processor.h |  31 +++++++++
> > > > >  arch/riscv/include/asm/smp.h       |  14 ++++
> > > > >  arch/riscv/kernel/cpu.c            |   5 ++
> > > > >  arch/riscv/kernel/cpufeature.c     |   1 +
> > > > >  arch/riscv/kernel/process.c        | 102 ++++++++++++++++++++++++++++-
> > > > >  arch/riscv/kernel/smp.c            |  39 +++++++----
> > > > >  9 files changed, 205 insertions(+), 23 deletions(-)
> > > > >
> > > > > --
> > > > > 2.20.1
> > > > >
> > > > >
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