[PATCH] local.conf.sample: disable prelink

[PATCH] local.conf.sample: disable prelink

[Alexander Kanavin]

There's an issue on ppc32 where one of the wayland libraries is incorrectly
relocated [1]. I also believe that any such custom, yocto-specific optimization
which does intricate binary rewriting needs to be justified with rigoroous
benchmarks but I can't find any.

https://bugzilla.yoctoproject.org/show_bug.cgi?id=14429

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
---
 meta-poky/conf/local.conf.sample | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/meta-poky/conf/local.conf.sample b/meta-poky/conf/local.conf.sample
index fb14379d28..1794126672 100644
--- a/meta-poky/conf/local.conf.sample
+++ b/meta-poky/conf/local.conf.sample
@@ -148,8 +148,7 @@ EXTRA_IMAGE_FEATURES ?= "debug-tweaks"
 # enable extra features. Some available options which can be included in this variable
 # are:
 #   - 'buildstats' collect build statistics
-#   - 'image-prelink' in order to prelink the filesystem image
-USER_CLASSES ?= "buildstats image-prelink"
+USER_CLASSES ?= "buildstats"
 
 #
 # Runtime testing of images
-- 
2.31.1

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Richard Purdie]

On Tue, 2021-06-15 at 10:12 +0200, Alexander Kanavin wrote:
> There's an issue on ppc32 where one of the wayland libraries is incorrectly
> relocated [1]. I also believe that any such custom, yocto-specific optimization
> which does intricate binary rewriting needs to be justified with rigoroous
> benchmarks but I can't find any.
> 
> https://bugzilla.yoctoproject.org/show_bug.cgi?id=14429
> 
> Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
> ---
>  meta-poky/conf/local.conf.sample | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

I appreciate the desire simply to delete/disable anything that causes issues
but in this case I draw the line and my answer is no. It works fine in the
vast majority of usage.

Cheers,

Richard

> diff --git a/meta-poky/conf/local.conf.sample b/meta-poky/conf/local.conf.sample
> index fb14379d28..1794126672 100644
> --- a/meta-poky/conf/local.conf.sample
> +++ b/meta-poky/conf/local.conf.sample
> @@ -148,8 +148,7 @@ EXTRA_IMAGE_FEATURES ?= "debug-tweaks"
>  # enable extra features. Some available options which can be included in this variable
>  # are:
>  #   - 'buildstats' collect build statistics
> -#   - 'image-prelink' in order to prelink the filesystem image
> -USER_CLASSES ?= "buildstats image-prelink"
> +USER_CLASSES ?= "buildstats"
>  
> 
> 
> 
>  #
>  # Runtime testing of images
> 
> 

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Alexander Kanavin]

[-- Attachment #1: Type: text/plain, Size: 467 bytes --]

On Tue, 15 Jun 2021 at 10:48, Richard Purdie <
richard.purdie@linuxfoundation.org> wrote:

> I appreciate the desire simply to delete/disable anything that causes
> issues
> but in this case I draw the line and my answer is no. It works fine in the
> vast majority of usage.
>

But where are the benchmarks that show it's actually beneficial? And
commitment from someone to maintain it and address open issues (there are
more than just this one)?

Alex

[-- Attachment #2: Type: text/html, Size: 832 bytes --]

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Richard Purdie]

On Tue, 2021-06-15 at 10:55 +0200, Alexander Kanavin wrote:
> On Tue, 15 Jun 2021 at 10:48, Richard Purdie <richard.purdie@linuxfoundation.org> wrote:
> > I appreciate the desire simply to delete/disable anything that causes issues
> > but in this case I draw the line and my answer is no. It works fine in the
> > vast majority of usage.
> > 
> 
> 
> But where are the benchmarks that show it's actually beneficial? 

Where are the benchmarks showing nearly any change we make is 
beneficial? The recently smp enabling for qemu don't have any,
nor do many other changes. We don't have the people/resources to
require benchmarks for everything like that.

> And commitment from someone to maintain it and address open issues 
> (there are more than just this one)?

We have a ton of areas of the project with this problem, for example
pseudo or eSDK, or even devtool and all it's bugs. We're not removing 
them and we're not removing this.

Worst case, it has to go onto the list of last resort which is I 
get around to fixing it, again, when I have time. You'll note from 
the history I have some previous experience at least :/.

Cheers,

Richard

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Alexander Kanavin]

[-- Attachment #1: Type: text/plain, Size: 2646 bytes --]

On Tue, 15 Jun 2021 at 10:55, Alexander Kanavin <alex.kanavin@gmail.com>
wrote:

> On Tue, 15 Jun 2021 at 10:48, Richard Purdie <
> richard.purdie@linuxfoundation.org> wrote:
>
>> I appreciate the desire simply to delete/disable anything that causes
>> issues
>> but in this case I draw the line and my answer is no. It works fine in the
>> vast majority of usage.
>>
>
> But where are the benchmarks that show it's actually beneficial? And
> commitment from someone to maintain it and address open issues (there are
> more than just this one)?
>

I went ahead and ran some quick benchmarks myself. Specifically:

1. Running 'free' after things have settled down at boot:

a) without prelink
core-image-sato-sdk
               total        used        free      shared  buff/cache
available
Mem:          489352      152104      236284         472      100964
 323824
core-image-ptest-fast
              total        used        free      shared  buff/cache
available
Mem:        1004680       43456      927688         256       33536
 941156

b) with prelink
core-image-sato-sdk
               total        used        free      shared  buff/cache
available
Mem:          489352      153048      235544         468      100760
 322900
core-image-ptest-fast
              total        used        free      shared  buff/cache
available
Mem:        1004680       44444      928128         256       32108
 940168

2. Running -c testimage

a) without prelink
core-image-sato-sdk () - Ran 66 tests in 96.693s
core-image-sato-sdk () - Ran 66 tests in 96.469s
core-image-sato-sdk () - Ran 66 tests in 94.994s
core-image-ptest-fast () - Ran 66 tests in 583.767s
core-image-ptest-fast () - Ran 66 tests in 576.564s
core-image-ptest-fast () - Ran 66 tests in 576.797s

b) with prelink
core-image-sato-sdk () - Ran 66 tests in 96.390s
core-image-sato-sdk () - Ran 66 tests in 96.615s
core-image-sato-sdk () - Ran 66 tests in 95.596s
core-image-ptest-fast () - Ran 66 tests in 576.248s
core-image-ptest-fast () - Ran 66 tests in 574.618s
core-image-ptest-fast () - Ran 66 tests in 576.760s

So the memory usage is actually *better* without prelink. And any timing
benefits are lost in statistical noise, in these tests at least.

So I do not think it is wrong to question the usefulness of this feature.
I'd like to hear Mark's take on this, as prelink-cross is primarily his
work, he's put a lot of effort into it, and I would want to know where the
benefits are. Note that Red Hat abandoned prelink in 2013, and
prelink-cross likewise hasn't seen any commits for two years.

Alex

[-- Attachment #2: Type: text/html, Size: 4307 bytes --]

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Khem Raj]

On 6/15/21 1:12 AM, Alexander Kanavin wrote:
> There's an issue on ppc32 where one of the wayland libraries is incorrectly
> relocated [1]. I also believe that any such custom, yocto-specific optimization
> which does intricate binary rewriting needs to be justified with rigoroous
> benchmarks but I can't find any.

We only use prelink on glibc based systems but they do give slight 
improved runtime/boottime performance on kind of devices yocto project 
is used on. However it will be good to get some idea of what this saving 
is now a days before we disable it fully.

its perhaps better to disable it for ppc alone so something like this
USER_CLASSES_remove = "image-prelink" in qemuppc.conf would be limited

> 
> https://bugzilla.yoctoproject.org/show_bug.cgi?id=14429
> 
> Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
> ---
>   meta-poky/conf/local.conf.sample | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/meta-poky/conf/local.conf.sample b/meta-poky/conf/local.conf.sample
> index fb14379d28..1794126672 100644
> --- a/meta-poky/conf/local.conf.sample
> +++ b/meta-poky/conf/local.conf.sample
> @@ -148,8 +148,7 @@ EXTRA_IMAGE_FEATURES ?= "debug-tweaks"
>   # enable extra features. Some available options which can be included in this variable
>   # are:
>   #   - 'buildstats' collect build statistics
> -#   - 'image-prelink' in order to prelink the filesystem image
> -USER_CLASSES ?= "buildstats image-prelink"
> +USER_CLASSES ?= "buildstats"
>   
>   #
>   # Runtime testing of images
> 
> 
> 
> 
> 

Re: [yocto] [poky] [PATCH] local.conf.sample: disable prelink

[Khem Raj]

On 6/15/21 8:21 AM, Alexander Kanavin wrote:
> 
> 
> On Tue, 15 Jun 2021 at 10:55, Alexander Kanavin <alex.kanavin@gmail.com 
> <mailto:alex.kanavin@gmail.com>> wrote:
> 
>     On Tue, 15 Jun 2021 at 10:48, Richard Purdie
>     <richard.purdie@linuxfoundation.org
>     <mailto:richard.purdie@linuxfoundation.org>> wrote:
> 
>         I appreciate the desire simply to delete/disable anything that
>         causes issues
>         but in this case I draw the line and my answer is no. It works
>         fine in the
>         vast majority of usage.
> 
> 
>     But where are the benchmarks that show it's actually beneficial? And
>     commitment from someone to maintain it and address open issues
>     (there are more than just this one)?
> 
> 
> I went ahead and ran some quick benchmarks myself. Specifically:
> 
> 1. Running 'free' after things have settled down at boot:
> 
> a) without prelink
> core-image-sato-sdk
>                 total        used        free      shared  buff/cache   
> available
> Mem:          489352      152104      236284         472      100964     
>   323824
> core-image-ptest-fast
>                total        used        free      shared  buff/cache   
> available
> Mem:        1004680       43456      927688         256       33536     
>   941156
> 
> b) with prelink
> core-image-sato-sdk
>                 total        used        free      shared  buff/cache   
> available
> Mem:          489352      153048      235544         468      100760     
>   322900
> core-image-ptest-fast
>                total        used        free      shared  buff/cache   
> available
> Mem:        1004680       44444      928128         256       32108     
>   940168
> 
> 2. Running -c testimage
> 
> a) without prelink
> core-image-sato-sdk () - Ran 66 tests in 96.693s
> core-image-sato-sdk () - Ran 66 tests in 96.469s
> core-image-sato-sdk () - Ran 66 tests in 94.994s
> core-image-ptest-fast () - Ran 66 tests in 583.767s
> core-image-ptest-fast () - Ran 66 tests in 576.564s
> core-image-ptest-fast () - Ran 66 tests in 576.797s
> 
> b) with prelink
> core-image-sato-sdk () - Ran 66 tests in 96.390s
> core-image-sato-sdk () - Ran 66 tests in 96.615s
> core-image-sato-sdk () - Ran 66 tests in 95.596s
> core-image-ptest-fast () - Ran 66 tests in 576.248s
> core-image-ptest-fast () - Ran 66 tests in 574.618s
> core-image-ptest-fast () - Ran 66 tests in 576.760s
>

I think the advantage is not on high end CPUs but more on less powerful 
ones, so perhaps trying it on something like rpi0 or lower would be good


> So the memory usage is actually *better* without prelink. And any timing 
> benefits are lost in statistical noise, in these tests at least.
> 
> So I do not think it is wrong to question the usefulness of this 
> feature. I'd like to hear Mark's take on this, as prelink-cross is 
> primarily his work, he's put a lot of effort into it, and I would want 
> to know where the benefits are. Note that Red Hat abandoned prelink in 
> 2013, and prelink-cross likewise hasn't seen any commits for two years.
> 



> Alex
> 
> 
> 
> 

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Richard Purdie]

On Tue, 2021-06-15 at 17:21 +0200, Alexander Kanavin wrote:
> So the memory usage is actually *better* without prelink. And any timing 
> benefits are lost in statistical noise, in these tests at least. 

The numbers certainly don't look convincing, thanks for running the tests.

> So I do not think it is wrong to question the usefulness of this feature. 
> I'd like to hear Mark's take on this, as prelink-cross is primarily his 
> work, he's put a lot of effort into it, and I would want to know where 
> the benefits are. Note that Red Hat abandoned prelink in 2013, and 
> prelink-cross likewise hasn't seen any commits for two years.

Mark and I did briefly talk yesterday and whilst Mark can/will speak
up, the summary was "drop it".

What makes me sad is that this is an embedded focused feature which 
the project should be caring about, i.e. memory footprint and speed.
I don't know why the numbers don't show it, Mark and I have theories
but it would take work to explore them and neither of us have that time,
nor are we experts on prelink able to maintain it with the time we have
available.

In my view the project should be caring about things like this, instead 
it looks more like a race to look like 'any other desktop distro' and 
drop anything which looks different.

I've tried to say no. Nobody else would appear to care, I'm hearing 
silence. I can't fight this.

Cheers,

Richard

Re: [poky] [PATCH] local.conf.sample: disable prelink

[Alexander Kanavin]

[-- Attachment #1: Type: text/plain, Size: 1070 bytes --]

On Wed, 16 Jun 2021 at 11:11, Richard Purdie <
richard.purdie@linuxfoundation.org> wrote:

> What makes me sad is that this is an embedded focused feature which
> the project should be caring about, i.e. memory footprint and speed.
> I don't know why the numbers don't show it, Mark and I have theories
> but it would take work to explore them and neither of us have that time,
> nor are we experts on prelink able to maintain it with the time we have
> available.
>

What I think happened is that glibc simply got better at linking, optimised
the runtime bits to the fullest, and any benefits prelink may once have had
aren't there anymore. Which is IMO fine - I am terrified of custom address
arithmetic that's not really understood by anyone, and very easy to get
wrong, with devastating consequences. I spent more or less the whole day
stepping assembly in gdb to figure out what exactly is going wrong in that
ppc bug.

The value proposition of Yocto is still very strong, and is not at all
affected by dropping prelink, in my opinion.

Alex

[-- Attachment #2: Type: text/html, Size: 1452 bytes --]

Re: [yocto] [poky] [PATCH] local.conf.sample: disable prelink

[Robert Berger]

Hi Alex, RP, Mark,

I did some research on the subject in order to try to figure out what is 
going on.

1) I come to a similar conclusion with what found, but tried to look a 
bit deeper for the reason.

1.1) The reason that cross-prelink is not prelinking is, that for a 
quite some time by default everything is built with PIE mode by default 
and cross-prelink does not seem to be able to work on exe/libs compiled 
with PIE mode. So seeing the same behavior with and without prelinking 
is what I would expect as long as everything is compiled with PIE mode.

A more detailed analysis of my tests can be found on my not yet 
officially published site:

https://rlbl.me/prelink-1

https://rlbl.me/prelink-2

Alex:

Can you please rebuild your test images without PIE mode and re-run the 
tests?

Then we should have the 4 test cases:

prelinked-with-pie
no-prelink-with-pie
prelink-no-pie
no-prelink-no-pie

I guess then we can discuss what are the next steps.

In my opinion the current default settings, which compile close to 
everything in PIE mode, but invoke also cross-prelink do not make much 
sense.

The question is: "Do we want to drop cross-prelink, or do we want to 
drag it along and come up more fine-grained configuration options?"

We could e.g. exclude certain files from pre-linking.

IMHO cross-prelink still works, but not on exe/libs which were compiled 
in PIE mode.

Regards,

Robert

Re: [yocto] [poky] [PATCH] local.conf.sample: disable prelink

[Alexander Kanavin]

[-- Attachment #1: Type: text/plain, Size: 2153 bytes --]

PIE is nowadays more or less the only available option and is expected for
improved security; Yocto does not even test non-PIE builds or provide an
off the shelf way to turn it off.

I also have to note that prelink does show higher RAM consumption in your
tests as well (MemFree column). On the constrained systems which would
benefit most from improved prelink timings that might be a bigger loss than
not prelinking.

But yes, there is a timing benefit visible in the tests: 0.01s vs 0.1s.

Alex

On Mon, 19 Jul 2021 at 22:58, Robert Berger@yocto.user <
robert.berger.yocto.user@gmail.com> wrote:

> Hi Alex, RP, Mark,
>
> I did some research on the subject in order to try to figure out what is
> going on.
>
> 1) I come to a similar conclusion with what found, but tried to look a
> bit deeper for the reason.
>
> 1.1) The reason that cross-prelink is not prelinking is, that for a
> quite some time by default everything is built with PIE mode by default
> and cross-prelink does not seem to be able to work on exe/libs compiled
> with PIE mode. So seeing the same behavior with and without prelinking
> is what I would expect as long as everything is compiled with PIE mode.
>
> A more detailed analysis of my tests can be found on my not yet
> officially published site:
>
> https://rlbl.me/prelink-1
>
> https://rlbl.me/prelink-2
>
> Alex:
>
> Can you please rebuild your test images without PIE mode and re-run the
> tests?
>
> Then we should have the 4 test cases:
>
> prelinked-with-pie
> no-prelink-with-pie
> prelink-no-pie
> no-prelink-no-pie
>
> I guess then we can discuss what are the next steps.
>
> In my opinion the current default settings, which compile close to
> everything in PIE mode, but invoke also cross-prelink do not make much
> sense.
>
> The question is: "Do we want to drop cross-prelink, or do we want to
> drag it along and come up more fine-grained configuration options?"
>
> We could e.g. exclude certain files from pre-linking.
>
> IMHO cross-prelink still works, but not on exe/libs which were compiled
> in PIE mode.
>
> Regards,
>
> Robert
>

[-- Attachment #2: Type: text/html, Size: 2803 bytes --]

Re: [yocto] [poky] [PATCH] local.conf.sample: disable prelink

[Robert Berger]

On 22/07/2021 22:05, Alexander Kanavin wrote:
> PIE is nowadays more or less the only available option and is expected 
> for improved security; Yocto does not even test non-PIE builds or 
> provide an off the shelf way to turn it off.

I am worried about those libraries, which are non-PIE libraries by 
default. My theory is, that they are non-PIE since prelink is able to 
operate on them. So prelink can "at least" be used a PIE detector.

They are:

lib/libdl-2.33.so is prelinked
lib/ld-2.33.so is prelinked
lib/libpthread-2.33.so is prelinked
lib/libc-2.33.so is prelinked

Is there are rational explanation why they are not compiled in PIE mode 
and/or if they are compiled in PIE mode how cross-prelink can operate on 
them? If cross-prelink can operate on them why not on the others?

> 
> I also have to note that prelink does show higher RAM consumption in 
> your tests as well (MemFree column). On the constrained systems which 
> would benefit most from improved prelink timings that might be a bigger 
> loss than not prelinking.

I guess we agree that MemFree shows free physical memory (user and 
kernel space).

My experiments show, that non-PIE and no prelink leaves the biggest 
amount of free physical memory.

They also show that non-PIE and prelink leave the smallest amount of 
free physical memory ;)

The difference is significant
prelinked-no-pie/no-prelink-no-pie:		4552 (kB)

If we leave things are they are:
prelinked-no-pie/prelinked-with-pie:		3972 (kB)

If we disable prelink (as you suggest - and I tend to agree since it 
does not make sense as it is right now)
prelinked-no-pie/no-prelink-with-pie:		4120 (kB)

...

but

if you look at the next line MemAvailable kB things looks a bit differently.

My interpretation of MemAvailable is, that it is an estimate of virtual 
memory available after reclaimable parts of memory (caches, buffer, 
slab,...) have been reclaimed without getting swap involved.

I see this:

MemAvailable kB

prelinked-with-pie 	939412
no-prelink-with-pie	939696
prelinked-no-pie	940344
no-prelink-no-pie	941216

Which means, that our current default setting is the worst possible 
solution ;)

no-prelink-no-pie would (theoretically) be the best.

I will try to update my second article and try to explain a bit more my 
interpretation of the results and maybe also try to see what bootchart 
says to all this.

Don't get me wrong. I am neither pro nor con prelink. I just would like 
to understand what it does, if it does something ;)

I spent quite some time on this - also discussing with most of you offline.
If you ask me, we should use your patch, since people didn't even notice 
that prelink can not prelink on PIE binaries for a couple of years.

So there does not seem to be much demand for it ;)

We can keep a "placebo" in for the homeopaths who think they use prelink 
in their images since PIE was enabled ;)

> 
> But yes, there is a timing benefit visible in the tests: 0.01s vs 0.1s.

Also less CPU usage can be seen. I hope I'll find time to run some test 
with bootchart. Maybe then we can also see boot time, memory, CPU,...

Regards,

Robert

> 
> Alex
> 
> On Mon, 19 Jul 2021 at 22:58, Robert Berger@yocto.user 
> <robert.berger.yocto.user@gmail.com 
> <mailto:robert.berger.yocto.user@gmail.com>> wrote:
> 
>     Hi Alex, RP, Mark,
> 
>     I did some research on the subject in order to try to figure out
>     what is
>     going on.
> 
>     1) I come to a similar conclusion with what found, but tried to look a
>     bit deeper for the reason.
> 
>     1.1) The reason that cross-prelink is not prelinking is, that for a
>     quite some time by default everything is built with PIE mode by default
>     and cross-prelink does not seem to be able to work on exe/libs compiled
>     with PIE mode. So seeing the same behavior with and without prelinking
>     is what I would expect as long as everything is compiled with PIE mode.
> 
>     A more detailed analysis of my tests can be found on my not yet
>     officially published site:
> 
>     https://rlbl.me/prelink-1 <https://rlbl.me/prelink-1>
> 
>     https://rlbl.me/prelink-2 <https://rlbl.me/prelink-2>
> 
>     Alex:
> 
>     Can you please rebuild your test images without PIE mode and re-run the
>     tests?
> 
>     Then we should have the 4 test cases:
> 
>     prelinked-with-pie
>     no-prelink-with-pie
>     prelink-no-pie
>     no-prelink-no-pie
> 
>     I guess then we can discuss what are the next steps.
> 
>     In my opinion the current default settings, which compile close to
>     everything in PIE mode, but invoke also cross-prelink do not make much
>     sense.
> 
>     The question is: "Do we want to drop cross-prelink, or do we want to
>     drag it along and come up more fine-grained configuration options?"
> 
>     We could e.g. exclude certain files from pre-linking.
> 
>     IMHO cross-prelink still works, but not on exe/libs which were compiled
>     in PIE mode.
> 
>     Regards,
> 
>     Robert
>