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

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Date:   Sat, 16 Sep 2023 01:16:30 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Stuebner <heiko@...ech.de>,
        Ley Foon Tan <leyfoon.tan@...rfivetech.com>,
        Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
        Conor Dooley <conor.dooley@...rochip.com>,
        David Laight <David.Laight@...lab.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        linux-riscv@...ts.infradead.org,
        Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH] RISC-V: Probe misaligned access speed in parallel

Yo Evan,

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.
> 
> Signed-off-by: Evan Green <evan@...osinc.com>
> 
> ---
> 
> 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.

Ya know, I think there's scope to add Reported-by:, Closes: and Fixes:
tags to this patch, mentioning explicitly that this has regressed boot
time for many core systems, so that this can be fixes material. What do
you think?

> ---
>  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;
> +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;

If you did something like s/measured/complete/ I think you could drop
the comment. Tis whatever though :)

Conor.

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