Re: [PATCH 0/3] riscv: Add PARAVIRT_SPINLOCKS support

[Andrew Jones <ajones@ventanamicro.com>]

On Mon, Dec 23, 2024 at 01:44:50PM +0530, Anup Patel wrote:
> On Sun, Dec 22, 2024 at 9:09 AM <guoren@kernel.org> wrote:
> >
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > Paravirtualized spinlocks allow a unfair qspinlock to replace the
> > ticket-lock or native fair qspinlock implementation with something
> > virtualization-friendly, for example, halt the virtual CPU rather
> > than spinning.
> >
> > You could observe the paravirt qspinlock internal work situation with
> > /sys/kernel/debug/tracing/trace:
> >
> > ls /sys/kernel/debug/tracing/events/paravirt/
> >  enable   filter   pv_kick  pv_wait
> >
> > echo 1 > /sys/kernel/debug/tracing/events/paravirt/enable
> > cat /sys/kernel/debug/tracing/trace
> >  entries-in-buffer/entries-written: 33927/33927   #P:12
> >
> >                                 _-----=> irqs-off/BH-disabled
> >                                / _----=> need-resched
> >                               | / _---=> hardirq/softirq
> >                               || / _--=> preempt-depth
> >                               ||| / _-=> migrate-disable
> >                               |||| /     delay
> >            TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> >               | |         |   |||||     |         |
> >              sh-100     [001] d..2.    28.312294: pv_wait: cpu 1 out of wfi
> >          <idle>-0       [000] d.h4.    28.322030: pv_kick: cpu 0 kick target cpu 1
> >              sh-100     [001] d..2.    30.982631: pv_wait: cpu 1 out of wfi
> >          <idle>-0       [000] d.h4.    30.993289: pv_kick: cpu 0 kick target cpu 1
> >              sh-100     [002] d..2.    44.987573: pv_wait: cpu 2 out of wfi
> >          <idle>-0       [000] d.h4.    44.989000: pv_kick: cpu 0 kick target cpu 2
> >          <idle>-0       [003] d.s3.    51.593978: pv_kick: cpu 3 kick target cpu 4
> >       rcu_sched-15      [004] d..2.    51.595192: pv_wait: cpu 4 out of wfi
> > lock_torture_wr-115     [004] ...2.    52.656482: pv_kick: cpu 4 kick target cpu 2
> > lock_torture_wr-113     [002] d..2.    52.659146: pv_wait: cpu 2 out of wfi
> > lock_torture_wr-114     [008] d..2.    52.659507: pv_wait: cpu 8 out of wfi
> > lock_torture_wr-114     [008] d..2.    52.663503: pv_wait: cpu 8 out of wfi
> > lock_torture_wr-113     [002] ...2.    52.666128: pv_kick: cpu 2 kick target cpu 8
> > lock_torture_wr-114     [008] d..2.    52.667261: pv_wait: cpu 8 out of wfi
> > lock_torture_wr-114     [009] .n.2.    53.141515: pv_kick: cpu 9 kick target cpu 11
> > lock_torture_wr-113     [002] d..2.    53.143339: pv_wait: cpu 2 out of wfi
> > lock_torture_wr-116     [007] d..2.    53.143412: pv_wait: cpu 7 out of wfi
> > lock_torture_wr-118     [000] d..2.    53.143457: pv_wait: cpu 0 out of wfi
> > lock_torture_wr-115     [008] d..2.    53.143481: pv_wait: cpu 8 out of wfi
> > lock_torture_wr-117     [011] d..2.    53.143522: pv_wait: cpu 11 out of wfi
> > lock_torture_wr-117     [011] ...2.    53.143987: pv_kick: cpu 11 kick target cpu 8
> > lock_torture_wr-115     [008] ...2.    53.144269: pv_kick: cpu 8 kick target cpu 7
> >
> > This series is split from [1].
> >
> > [1]: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@kernel.org/
> >
> > Guo Ren (3):
> >   RISC-V: paravirt: Add pvqspinlock KVM backend
> >   RISC-V: paravirt: Add pvqspinlock frontend
> >   RISC-V: paravirt: pvqspinlock: Add trace point for pv_kick/wait
> 
> This should be an RFC series until the corresponding SBI extension
> is frozen. AFAIK, no one has posted any SBI extension in this context
> on any of the RVI mailing lists.
> 
> Please work with the PRS TG and Hypervisors SIG for the proposed
> SBI extension.
>

I promised Guo I'd help out by writing a draft for the SBI extension. I'll
try to get something posted this week.

Thanks,
drew