Re: [PATCH v2] RISC-V: Probe misaligned access speed in parallel

[Conor Dooley]

On Wed, Sep 20, 2023 at 03:06:45PM -0700, Evan Green wrote:
> On Wed, Sep 20, 2023 at 2:27 PM Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> >
> > On Wed, Sep 20, 2023 at 2:04 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> > >
> > > Yo,
> > >
> > > On Wed, Sep 20, 2023 at 12:38:01PM -0700, Evan Green wrote:
> > > > Probing for misaligned access speed takes about 0.06 seconds. On a
> > > > system with 64 cores, doing this in smp_callin() means it's done
> > > > serially, extending boot time by 3.8 seconds. That's a lot of boot time.
> > > >
> > > > Instead of measuring each CPU serially, let's do the measurements on
> > > > all CPUs in parallel. If we disable preemption on all CPUs, the
> > > > jiffies stop ticking, so we can do this in stages of 1) everybody
> > > > except core 0, then 2) core 0.
> > > >
> > > > The measurement call in smp_callin() stays around, but is now
> > > > conditionalized to only run if a new CPU shows up after the round of
> > > > in-parallel measurements has run. The goal is to have the measurement
> > > > call not run during boot or suspend/resume, but only on a hotplug
> > > > addition.
> > > >
> > > > Reported-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > > > Closes: https://lore.kernel.org/all/mhng-9359993d-6872-4134-83ce-c97debe1cf9a@palmer-ri-x1c9/T/#mae9b8f40016f9df428829d33360144dc5026bcbf
> > > > Fixes: 584ea6564bca ("RISC-V: Probe for unaligned access speed")
> > > > Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
> > > > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > > > Tested-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > > >
> > > > ---
> > > >
> > > > Changes in v2:
> > > >  - Removed new global, used system_state == SYSTEM_RUNNING instead
> > > >    (Jisheng)
> > > >  - Added tags
> > > >
> > > >  arch/riscv/include/asm/cpufeature.h |  2 +-
> > > >  arch/riscv/kernel/cpufeature.c      | 22 +++++++++++++++++-----
> > > >  arch/riscv/kernel/smpboot.c         | 11 ++++++++++-
> > > >  3 files changed, 28 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > > > index d0345bd659c9..b139796392d0 100644
> > > > --- a/arch/riscv/include/asm/cpufeature.h
> > > > +++ b/arch/riscv/include/asm/cpufeature.h
> > > > @@ -30,6 +30,6 @@ DECLARE_PER_CPU(long, misaligned_access_speed);
> > > >  /* Per-cpu ISA extensions. */
> > > >  extern struct riscv_isainfo hart_isa[NR_CPUS];
> > > >
> > > > -void check_unaligned_access(int cpu);
> > > > +int check_unaligned_access(void *unused);
> > > >
> > > >  #endif
> > > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > > index 1cfbba65d11a..40bb854fcb96 100644
> > > > --- a/arch/riscv/kernel/cpufeature.c
> > > > +++ b/arch/riscv/kernel/cpufeature.c
> > > > @@ -556,8 +556,9 @@ unsigned long riscv_get_elf_hwcap(void)
> > > >       return hwcap;
> > > >  }
> > > >
> > > > -void check_unaligned_access(int cpu)
> > > > +int check_unaligned_access(void *unused)
> > > >  {
> > > > +     int cpu = smp_processor_id();
> > > >       u64 start_cycles, end_cycles;
> > > >       u64 word_cycles;
> > > >       u64 byte_cycles;
> > > > @@ -571,7 +572,7 @@ void check_unaligned_access(int cpu)
> > > >       page = alloc_pages(GFP_NOWAIT, get_order(MISALIGNED_BUFFER_SIZE));
> > > >       if (!page) {
> > > >               pr_warn("Can't alloc pages to measure memcpy performance");
> > > > -             return;
> > > > +             return 0;
> > >
> > > Dumb question maybe, but I am limited setup wise at the moment due to
> > > a hardware failure which makes checking stuff hard, why the signature
> > > change? Requirement for on_each_cpu()?
> > >
> >
> > Requirement for smp_call_on_cpu.
> 
> Right.
> 
> >
> > > >       }
> > > >
> > > >       /* Make an unaligned destination buffer. */
> > > > @@ -643,15 +644,26 @@ void check_unaligned_access(int cpu)
> > > >
> > > >  out:
> > > >       __free_pages(page, get_order(MISALIGNED_BUFFER_SIZE));
> > > > +     return 0;
> > > > +}
> > > > +
> > > > +static void check_unaligned_access_nonboot_cpu(void *param)
> > > > +{
> > > > +     if (smp_processor_id() != 0)
> > > > +             check_unaligned_access(param);
> > > >  }
> > > >
> > > > -static int check_unaligned_access_boot_cpu(void)
> > > > +static int check_unaligned_access_all_cpus(void)
> > > >  {
> > > > -     check_unaligned_access(0);
> > > > +     /* Check everybody except 0, who stays behind to tend jiffies. */
> > > > +     on_each_cpu(check_unaligned_access_nonboot_cpu, NULL, 1);
> > > > +
> > > > +     /* Check core 0. */
> > > > +     smp_call_on_cpu(0, check_unaligned_access, NULL, true);
> > > >       return 0;
> > >
> > > Why does this function return an int if it can only return 0?
> > >
> 
> This is a requirement on the initcall_t function pointer type.

Ahh great, thanks for the explanations!

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