linux-kernel - Re: [v1 0/5] allow to reserve memory for normal kexec kernel

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Date:   Mon, 8 Jul 2019 20:09:04 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        kexec@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        corbet@....net, Catalin Marinas <catalin.marinas@....com>,
        will@...nel.org, linux-doc@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v1 0/5] allow to reserve memory for normal kexec kernel

> Something is very very wrong there.
>
> Last I measured memory bandwidth seriously I could touch a Gigabyte per
> second easily, and that was nearly 20 years ago.  Did you manage to
> disable caching or have some particularly slow code that does the
> reolocations.
>
> There is a serious cost to reserving memory in that it is simply not
> available at other times.  For kexec on panic there is no other reliable
> way to get memory that won't be DMA'd to.

Hi Eric,

Thank you for your comments.

Indeed, but sometimes fast reboot is more important than the cost of
reserving 32M-64M of memory.

>
> We have options in this case and I would strongly encourage you to track
> down why that copy in relocation is so very slow.  I suspect a 4KiB page
> size is large enough that it can swamp pointer following costs.
>
> My back of the napkin math says even 20 years ago your copying costs
> should be only 0.037s.  The only machine I have ever tested on where
> the copy costs were noticable was my old 386.
>
> Maybe I am out to lunch here but a claim that your memory only runs
> at 100MiB/s (the speed of my spinning rust hard drive) is rather
> incredible.

I agree,  my measurement on this machine was 2,857MB/s. Perhaps when
MMU is disabled ARM64 also has caching disabled? The function that
loops through array of pages and relocates them to final destination
is this:

https://soleen.com/source/xref/linux/arch/arm64/kernel/relocate_kernel.S?r=d2912cb1#29

A comment before calling it:

205   /*
206   * cpu_soft_restart will shutdown the MMU, disable data caches, then
207   * transfer control to the reboot_code_buffer which contains a copy of
208   * the arm64_relocate_new_kernel routine.  arm64_relocate_new_kernel
209   * uses physical addressing to relocate the new image to its final
210   * position and transfers control to the image entry point when the
211   * relocation is complete.
212   * In kexec case, kimage->start points to purgatory assuming that
213   * kernel entry and dtb address are embedded in purgatory by
214   * userspace (kexec-tools).
215   * In kexec_file case, the kernel starts directly without purgatory.
216   */
https://soleen.com/source/xref/linux/arch/arm64/kernel/machine_kexec.c?r=d2912cb1#206

So, as I understand at least data caches are disabled, and MMU is
disabled, perhaps this is why this function is so incredibly slow?

Perhaps, there is a better way to fix this problem by keeping caches
enabled while still relocating? Any suggestions from Aarch64
developers?

Pasha