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

[Andrew Jones]

On Fri, Sep 15, 2023 at 11:49:03AM -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.

Yay! I had just recently tested suspend/resume and wanted to report the
probe as an issue, but I hadn't gotten around to it. This patch resolves
the issue, so

Test-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>

> 
> Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
> 
> ---
> 
> Jisheng, I didn't add your Tested-by tag since the patch evolved from
> the one you tested. Hopefully this one brings you the same result.
> 
> ---
>  arch/riscv/include/asm/cpufeature.h |  3 ++-
>  arch/riscv/kernel/cpufeature.c      | 28 +++++++++++++++++++++++-----
>  arch/riscv/kernel/smpboot.c         | 11 ++++++++++-
>  3 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index d0345bd659c9..19e7817eba10 100644
> --- a/arch/riscv/include/asm/cpufeature.h
> +++ b/arch/riscv/include/asm/cpufeature.h
> @@ -30,6 +30,7 @@ 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);
> +extern bool misaligned_speed_measured;

Do we need this new state or could we just always check the boot cpu's
state to get the same information?

 per_cpu(misaligned_access_speed, 0) != RISCV_HWPROBE_MISALIGNED_UNKNOWN

> +int check_unaligned_access(void *unused);
>  
>  #endif
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 1cfbba65d11a..8eb36e1dfb95 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -42,6 +42,9 @@ struct riscv_isainfo hart_isa[NR_CPUS];
>  /* Performance information */
>  DEFINE_PER_CPU(long, misaligned_access_speed);
>  
> +/* Boot-time in-parallel unaligned access measurement has occurred. */
> +bool misaligned_speed_measured;
> +
>  /**
>   * riscv_isa_extension_base() - Get base extension word
>   *
> @@ -556,8 +559,9 @@ 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 +575,7 @@ 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;
>  	}
>  
>  	/* Make an unaligned destination buffer. */
> @@ -643,15 +647,29 @@ 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);
> +
> +	/* Boot-time measurements are complete. */
> +	misaligned_speed_measured = true;
>  	return 0;
>  }
>  
> -arch_initcall(check_unaligned_access_boot_cpu);
> +arch_initcall(check_unaligned_access_all_cpus);
>  
>  #ifdef CONFIG_RISCV_ALTERNATIVE
>  /*
> diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> index 1b8da4e40a4d..39322ae20a75 100644
> --- a/arch/riscv/kernel/smpboot.c
> +++ b/arch/riscv/kernel/smpboot.c
> @@ -27,6 +27,7 @@
>  #include <linux/sched/mm.h>
>  #include <asm/cpu_ops.h>
>  #include <asm/cpufeature.h>
> +#include <asm/hwprobe.h>
>  #include <asm/irq.h>
>  #include <asm/mmu_context.h>
>  #include <asm/numa.h>
> @@ -246,7 +247,15 @@ asmlinkage __visible void smp_callin(void)
>  
>  	numa_add_cpu(curr_cpuid);
>  	set_cpu_online(curr_cpuid, 1);
> -	check_unaligned_access(curr_cpuid);
> +
> +	/*
> +	 * Boot-time misaligned access speed measurements are done in parallel
> +	 * in an initcall. Only measure here for hotplug.
> +	 */
> +	if (misaligned_speed_measured &&
> +	    (per_cpu(misaligned_access_speed, curr_cpuid) == RISCV_HWPROBE_MISALIGNED_UNKNOWN)) {
> +		check_unaligned_access(NULL);
> +	}
>  
>  	if (has_vector()) {
>  		if (riscv_v_setup_vsize())
> -- 
> 2.34.1
>

Besides my reluctance to add another global variable, this looks good to
me.

Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>

Thanks,
drew