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

From: Pavel Tatashin
Date: Mon Jul 08 2019 - 20:09:19 EST

> 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:

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 */

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