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

From: Scott Wood <scottwood@freescale.com>
To: Pratyush Anand <panand@redhat.com>, James Morse <james.morse@arm.com>
Cc: Geoff Levand <geoff@infradead.org>,
	kexec@lists.infradead.org, Simon Horman <horms@verge.net.au>,
	Vivek Goyal <vgoyal@redhat.com>,
	Petitboot@lists.ozlabs.org, Dave Young <dyoung@redhat.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] kexec: Add --lite option
Date: Mon, 7 Dec 2015 19:03:20 -0600	[thread overview]
Message-ID: <1449536600.15946.102.camel@freescale.com> (raw)
In-Reply-To: <20151207140755.GG16406@dhcppc13.redhat.com>

On Mon, 2015-12-07 at 19:37 +0530, Pratyush Anand wrote:
> Hi James,
> 
> Thanks for the reply.
> 
> On 07/12/2015:01:16:06 PM, James Morse wrote:
> > Hi Pratyush,
> > 
> > On 07/12/15 11:48, Pratyush Anand wrote:
> > > > 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?
> > 
> > I haven't benchmarked this, but:
> > 
> > util_lib/sha256.c contains calls out to memcpy().
> > In your case 1, this will use the glibc version. In case 2, it will use
> > the version implemented in purgatory/string.c, which is a byte-by-byte
> > copy.
> > 
> 
> Yes, I agree that byte copy is too slow. But, memcpy() in sha256_update()
> will
> copy only few bytes (I think max 126 bytes). Most of the data will be
> processed
> using loop while( length >= 64 ){}, where we do not have any memcpy.So, I do
> not
> think that this would be causing such a difference.
> 
> Could it be the case that I am not using perfect memory attributes while
> setting
> up identity mapping and enabling D-cache. My implementation is here:
> https://github.com/pratyushanand/kexec-tools/commit/8efdbc56b52f99a8a074edd0
> ddc519d7b68be82f

FWIW, purgatory is fast for me on PPC (sub-second), so between that (assuming
it's not due to some PPC-specific optimization) and the fact that you don't
see any improvement with cache, I'd guess there's something wrong with how
you're enabling caches.

-Scott

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