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

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Message-ID: <20231103-uproar-rumbling-0a7a5ecb385e@spud>
Date:   Fri, 3 Nov 2023 08:34:38 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        David Laight <David.Laight@...lab.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrew Jones <ajones@...tanamicro.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Ley Foon Tan <leyfoon.tan@...rfivetech.com>,
        Marc Zyngier <maz@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Sunil V L <sunilvl@...tanamicro.com>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] RISC-V: Probe misaligned access speed in parallel

On Thu, Nov 02, 2023 at 03:41:58PM -0700, Evan Green wrote:
> On Fri, Sep 15, 2023 at 11:49 AM Evan Green <evan@...osinc.com> 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>
> 
> Shoot, I saw the other thread [1] where it seems like my use of
> alloc_pages() in this context is improper? I had thought I was
> alright, as Documentation/core-api/memory-allocation.rst says:
> 
>  > If the allocation is performed from an atomic context, e.g interrupt
>  > handler, use ``GFP_NOWAIT``.
> 
> Any tips for reproducing that splat? I have CONFIG_DEBUG_ATOMIC_SLEEP
> on (it's in the defconfig), and lockdep, and I'm on Conor's
> linux-6.6.y-rt, but so far I'm not seeing it.

It was originally produced in hardware, but I can also see these issues
in QEMU's emulation of my hardware (although as you may have seen, I get
them both with and without this patch). My qemu incantation was
something like:
	$(qemu) -M microchip-icicle-kit \
		-m 3G -smp 5 \
		-kernel vmlinux.bin \
		-dtb mpfs-icicle.dtb \
		-initrd initramfs \
		-display none -serial null \
		-serial stdio \
		-D qemu.log -d unimp

Where the kernel was built from the .config in that branch in my repo.

Cheers,
Conor.

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