Re: [PATCH] kexec: Add --lite option

[Pratyush Anand <panand@redhat.com>]

+linux-arm-kernel@lists.infradead.org (May be someone from arm kernel list can
give some more input)

On 04/11/2015:11:56:51 PM, Scott Wood wrote:
> On Thu, 2015-10-22 at 12:08 -0700, Geoff Levand wrote:
> > I notice the difference on the my arm64 system, so I guess we
> > are even on that.
> 
> For me it was beyond "notice the difference" -- I thought it was completely 
> broken, and was preparing to debug, until it started spitting out output over 
> a minute later.
> 
> Compiling the sha256 code with -O2 instead of -O0 cut it down to around 10 
> seconds (still unpleasant, but not quite as crazy... still unacceptable for 
> non-kdump, though).

Yes, compiling purgatory code with -O2 helps to improve the timing, and I notice
that enabling D-cache on top of -O2 does not improve it further. However, I am
still not able to understand that why there is huge difference between following
two.

1) When we execute kexec() system call in first kernel, at that time it
calculates sha256 on all the binaries [1]. It take almost un-noticeable time
(less than a sec) there.

2) When purgatory is executed then it re-calculates sha256 using same routines
[2] on same binary data as that of case (1). But, now it takes 10-20 sec
(depending of size of binaries)?

Why did not it take same time with O2 + D-cache enabled? I think, we should be
able to achieve same time in second case as well. What is missing?

~Pratyush

[1] http://git.kernel.org/cgit/utils/kernel/kexec/kexec-tools.git/tree/kexec/kexec.c#n650
[2] http://git.kernel.org/cgit/utils/kernel/kexec/kexec-tools.git/tree/purgatory/purgatory.c#n20

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