Re: [PATCH v14 2/5] riscv: Add static key for misaligned accesses

[Charlie Jenkins <charlie@rivosinc.com>]

On Mon, Jan 08, 2024 at 11:22:34AM -0800, Evan Green wrote:
> On Wed, Dec 27, 2023 at 9:38 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
> >
> > Support static branches depending on the value of misaligned accesses.
> > This will be used by a later patch in the series. All online cpus must
> > be considered "fast" for this static branch to be flipped.
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> 
> This is fancier than I would have gone for, I probably would have
> punted on heterogeneous hotplug out of laziness for now. However, what
> you've done looks smart, in that we'll basically flip the branch if at
> any moment all the online CPUs are fast. I've got some nits below, but
> won't withhold my review for them (making them optional I suppose :)).
> 
> Reviewed-by: Evan Green <evan@rivosinc.com>
> 

Thanks!

> > ---
> >  arch/riscv/include/asm/cpufeature.h |  2 +
> >  arch/riscv/kernel/cpufeature.c      | 89 +++++++++++++++++++++++++++++++++++--
> >  2 files changed, 87 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> > index a418c3112cd6..7b129e5e2f07 100644
> > --- a/arch/riscv/include/asm/cpufeature.h
> > +++ b/arch/riscv/include/asm/cpufeature.h
> > @@ -133,4 +133,6 @@ static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsi
> >         return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> >  }
> >
> > +DECLARE_STATIC_KEY_FALSE(fast_misaligned_access_speed_key);
> > +
> >  #endif
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index b3785ffc1570..dfd716b93565 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -8,8 +8,10 @@
> >
> >  #include <linux/acpi.h>
> >  #include <linux/bitmap.h>
> > +#include <linux/cpu.h>
> >  #include <linux/cpuhotplug.h>
> >  #include <linux/ctype.h>
> > +#include <linux/jump_label.h>
> >  #include <linux/log2.h>
> >  #include <linux/memory.h>
> >  #include <linux/module.h>
> > @@ -44,6 +46,8 @@ struct riscv_isainfo hart_isa[NR_CPUS];
> >  /* Performance information */
> >  DEFINE_PER_CPU(long, misaligned_access_speed);
> >
> > +static cpumask_t fast_misaligned_access;
> > +
> >  /**
> >   * riscv_isa_extension_base() - Get base extension word
> >   *
> > @@ -643,6 +647,16 @@ static int check_unaligned_access(void *param)
> >                 (speed == RISCV_HWPROBE_MISALIGNED_FAST) ? "fast" : "slow");
> >
> >         per_cpu(misaligned_access_speed, cpu) = speed;
> > +
> > +       /*
> > +        * Set the value of fast_misaligned_access of a CPU. These operations
> > +        * are atomic to avoid race conditions.
> > +        */
> > +       if (speed == RISCV_HWPROBE_MISALIGNED_FAST)
> > +               cpumask_set_cpu(cpu, &fast_misaligned_access);
> > +       else
> > +               cpumask_clear_cpu(cpu, &fast_misaligned_access);
> > +
> >         return 0;
> >  }
> >
> > @@ -655,13 +669,70 @@ static void check_unaligned_access_nonboot_cpu(void *param)
> >                 check_unaligned_access(pages[cpu]);
> >  }
> >
> > +DEFINE_STATIC_KEY_FALSE(fast_misaligned_access_speed_key);
> > +
> > +static int exclude_set_unaligned_access_static_branches(int cpu)
> > +{
> > +       /*
> > +        * Same as set_unaligned_access_static_branches, except excludes the
> > +        * given CPU from the result. When a CPU is hotplugged into an offline
> > +        * state, this function is called before the CPU is set to offline in
> > +        * the cpumask, and thus the CPU needs to be explicitly excluded.
> > +        */
> > +
> > +       cpumask_t online_fast_misaligned_access;
> > +
> > +       cpumask_and(&online_fast_misaligned_access, &fast_misaligned_access, cpu_online_mask);
> > +       cpumask_clear_cpu(cpu, &online_fast_misaligned_access);
> > +
> > +       if (cpumask_weight(&online_fast_misaligned_access) == (num_online_cpus() - 1))
> > +               static_branch_enable_cpuslocked(&fast_misaligned_access_speed_key);
> > +       else
> > +               static_branch_disable_cpuslocked(&fast_misaligned_access_speed_key);
> > +
> > +       return 0;
> > +}
> 
> A minor nit:  the function above and below are looking a little
> copy/pasty, and lead to multiple spots where the static branch gets
> changed. You could make a third function that actually does the
> setting with parameters, then these two could call it in different
> ways. The return types also don't need to be int, since you always
> return 0. Something like:
> 
> static void modify_unaligned_access_branches(cpumask_t *mask, int weight)
> {
>         if (cpumask_weight(mask) == weight) {
>                static_branch_enable_cpuslocked(&fast_misaligned_access_speed_key);
>         } else {
>                static_branch_disable_cpuslocked(&fast_misaligned_access_speed_key);
>         }
> }
> 
> static void set_unaligned_access_branches(void)
> {
>         cpumask_t fast_and_online;
> 
>         cpumask_and(&fast_and_online, &fast_misaligned_access, cpu_online_mask);
>         modify_unaligned_access_branches(&fast_and_online, num_online_cpus());
> }
> 
> static void set_unaligned_access_branches_except_cpu(unsigned int cpu)
> {
>         cpumask_t fast_except_me;
> 
>         cpumask_and(&online_fast_misaligned_access,
> &fast_misaligned_access, cpu_online_mask);
>         cpumask_clear_cpu(cpu, &fast_except_me);
>         modify_unaligned_access_branches(&fast_except_me,
> num_online_cpus() - 1);
> }
> 

Great suggestions, I will apply these changes and send out a new
version.

- Charlie

> > +
> > +static int set_unaligned_access_static_branches(void)
> > +{
> > +       /*
> > +        * This will be called after check_unaligned_access_all_cpus so the
> > +        * result of unaligned access speed for all CPUs will be available.
> > +        *
> > +        * To avoid the number of online cpus changing between reading
> > +        * cpu_online_mask and calling num_online_cpus, cpus_read_lock must be
> > +        * held before calling this function.
> > +        */
> > +       cpumask_t online_fast_misaligned_access;
> > +
> > +       cpumask_and(&online_fast_misaligned_access, &fast_misaligned_access, cpu_online_mask);
> > +
> > +       if (cpumask_weight(&online_fast_misaligned_access) == num_online_cpus())
> > +               static_branch_enable_cpuslocked(&fast_misaligned_access_speed_key);
> > +       else
> > +               static_branch_disable_cpuslocked(&fast_misaligned_access_speed_key);
> > +
> > +       return 0;
> > +}
> > +
> > +static int lock_and_set_unaligned_access_static_branch(void)
> > +{
> > +       cpus_read_lock();
> > +       set_unaligned_access_static_branches();
> > +       cpus_read_unlock();
> > +
> > +       return 0;
> > +}
> > +
> > +arch_initcall_sync(lock_and_set_unaligned_access_static_branch);
> > +
> >  static int riscv_online_cpu(unsigned int cpu)
> >  {
> >         static struct page *buf;
> >
> >         /* We are already set since the last check */
> >         if (per_cpu(misaligned_access_speed, cpu) != RISCV_HWPROBE_MISALIGNED_UNKNOWN)
> > -               return 0;
> > +               goto exit;
> >
> >         buf = alloc_pages(GFP_KERNEL, MISALIGNED_BUFFER_ORDER);
> >         if (!buf) {
> > @@ -671,7 +742,14 @@ static int riscv_online_cpu(unsigned int cpu)
> >
> >         check_unaligned_access(buf);
> >         __free_pages(buf, MISALIGNED_BUFFER_ORDER);
> > -       return 0;
> > +
> > +exit:
> > +       return set_unaligned_access_static_branches();
> > +}
> > +
> > +static int riscv_offline_cpu(unsigned int cpu)
> > +{
> > +       return exclude_set_unaligned_access_static_branches(cpu);
> >  }
> >
> >  /* Measure unaligned access on all CPUs present at boot in parallel. */
> > @@ -705,9 +783,12 @@ static int check_unaligned_access_all_cpus(void)
> >         /* Check core 0. */
> >         smp_call_on_cpu(0, check_unaligned_access, bufs[0], true);
> >
> > -       /* Setup hotplug callback for any new CPUs that come online. */
> > +       /*
> > +        * Setup hotplug callbacks for any new CPUs that come online or go
> > +        * offline.
> > +        */
> >         cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "riscv:online",
> > -                                 riscv_online_cpu, NULL);
> > +                                 riscv_online_cpu, riscv_offline_cpu);
> >
> >  out:
> >         unaligned_emulation_finish();
> >
> > --
> > 2.43.0
> >

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