Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 4 Jan 2022 11:47:30 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Ard Biesheuvel <ardb@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@zeniv.linux.org.uk>, llvm@lists.linux.dev
Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree
 -v1: Eliminate the Linux kernel's "Dependency Hell"
Message-ID: <YdQlwnDs2N9a5Reh@gmail.com>
References: <YdIfz+LMewetSaEB@gmail.com>
 <YdM4Z5a+SWV53yol@archlinux-ax161>
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* Nathan Chancellor <nathan@kernel.org> wrote:

> Hi Ingo,
> 
> On Sun, Jan 02, 2022 at 10:57:35PM +0100, Ingo Molnar wrote:
> > Before going into details about how this tree solves 'dependency hell' 
> > exactly, here's the current kernel build performance gain with 
> > CONFIG_FAST_HEADERS=y enabled, (and with CONFIG_KALLSYMS_FAST=y enabled as 
> > well - see below), using a stock x86 Linux distribution's .config with all 
> > modules built into the vmlinux:
> > 
> >   #
> >   # Performance counter stats for 'make -j96 vmlinux' (3 runs):
> >   #
> >   # (Elapsed time in seconds):
> >   #
> > 
> >   v5.16-rc7:            231.34 +- 0.60 secs, 15.5 builds/hour    # [ vanilla baseline ]
> >   -fast-headers-v1:     129.97 +- 0.51 secs, 27.7 builds/hour    # +78.0% improvement
> 
> This is really impressive; as someone who constantly builds large
> kernels for test coverage, I am excited about less time to get results.
> Testing on an 80-core arm64 server (the fastest machine I have access to
> at the moment) with LLVM, I can see anywhere from 18% to 35% improvement.
> 
> 
> Benchmark 1: ARCH=arm64 defconfig (linux)
>   Time (mean ± σ):     97.159 s ±  0.246 s    [User: 4828.383 s, System: 611.256 s]
>   Range (min … max):   96.900 s … 97.648 s    10 runs
> 
> Benchmark 2: ARCH=arm64 defconfig (linux-fast-headers)
>   Time (mean ± σ):     76.300 s ±  0.107 s    [User: 3149.986 s, System: 436.487 s]
>   Range (min … max):   76.117 s … 76.467 s    10 runs

That looks good, thanks for giving it a test, and thanks for all the fixes! 
:-)

Note that on ARM64 the elapsed time improvement is 'only' 18-35%, because 
the triple-linking of vmlinux serializes much of the of a build & ARM64 
doesn't have the kallsyms-objtool feature yet.

But we can already see how much faster it became, from the user+system time 
spent building the kernel:

           vanilla: 4828.383 s + 611.256 s = 5439.639 s
  -fast-headers-v1: 3149.986 s + 436.487 s = 3586.473 s

That's a +51% speedup. :-)

With CONFIG_KALLSYMS_FAST=y on x86, the final link gets faster by about 
60%-70%, so the header improvements will more directly show up in elapsed 
time as well.

Plus I spent more time looking at x86 header bloat than at ARM64 header 
bloat. In the end I think the improvement could probably moved into the 
broad 60-70% range that I see on x86.

All the other ARM64 tests show a 37%-43% improvement in CPU time used:

> Benchmark 1: ARCH=arm64 allmodconfig (linux)
>   Time (mean ± σ):     390.106 s ±  0.192 s    [User: 23893.382 s, System: 2802.413 s]
>   Range (min … max):   389.942 s … 390.513 s    7 runs
> 
> Benchmark 2: ARCH=arm64 allmodconfig (linux-fast-headers)
>   Time (mean ± σ):     288.066 s ±  0.621 s    [User: 16436.098 s, System: 2117.352 s]
>   Range (min … max):   287.131 s … 288.982 s    7 runs

# (23893.382+2802.413)/(16436.098+2117.352) = +43% in throughput.


> Benchmark 1: ARCH=arm64 allyesconfig (linux)
>   Time (mean ± σ):     557.752 s ±  1.019 s    [User: 21227.404 s, System: 2226.121 s]
>   Range (min … max):   555.833 s … 558.775 s    7 runs
> 
> Benchmark 2: ARCH=arm64 allyesconfig (linux-fast-headers)
>   Time (mean ± σ):     473.815 s ±  1.793 s    [User: 15351.991 s, System: 1689.630 s]
>   Range (min … max):   471.542 s … 476.830 s    7 runs

# (21227.404+2226.121)/(15351.991+1689.630) = +37%


> Benchmark 1: ARCH=x86_64 defconfig (linux)
>   Time (mean ± σ):     41.122 s ±  0.190 s    [User: 1700.206 s, System: 205.555 s]
>   Range (min … max):   40.966 s … 41.515 s    7 runs
> 
> Benchmark 2: ARCH=x86_64 defconfig (linux-fast-headers)
>   Time (mean ± σ):     36.357 s ±  0.183 s    [User: 1134.252 s, System: 152.396 s]
>   Range (min … max):   35.983 s … 36.534 s    7 runs


# (1700.206+205.555)/(1134.252+152.396) = +48%

> Summary
>   'ARCH=x86_64 defconfig (linux-fast-headers)' ran
>     1.13 ± 0.01 times faster than 'ARCH=x86_64 defconfig (linux)'

Now this x86-defconfig result you got is a bit weird - it *should* have 
been around ~50% faster on x86 in terms of elapsed time too.

Here's how x86-64 defconfig looks like on my system - with 128 GB RAM & 
fast NVDIMMs and 64 CPUs:

   #
   # -v5.16-rc8:
   #

   $ perf stat --repeat 3 -e instructions,cycles,cpu-clock --sync --pre "make clean >/dev/null" make -j96 vmlinux >/dev/null

   Performance counter stats for 'make -j96 vmlinux' (3 runs):

   4,906,953,379,372      instructions              #    0.90  insn per cycle           ( +-  0.00% )
   5,475,163,448,391      cycles                    #    3.898 GHz                      ( +-  0.01% )
        1,404,614.64 msec cpu-clock                 #   45.864 CPUs utilized            ( +-  0.01% )

             30.6258 +- 0.0337 seconds time elapsed  ( +-  0.11% )

   #
   # -fast-headers-v1:
   #

   $ make defconfig
   $ grep KALLSYMS_FAST .config
   CONFIG_KALLSYMS_FAST=y

   $ perf stat --repeat 3 -e instructions,cycles,cpu-clock --sync --pre "make clean >/dev/null" make -j96 vmlinux >/dev/null

    Performance counter stats for 'make -j96 vmlinux' (3 runs):

     3,500,079,269,120      instructions              #    0.90  insn per cycle           ( +-  0.00% )
     3,872,081,278,824      cycles                    #    3.895 GHz                      ( +-  0.10% )
            993,448.13 msec cpu-clock                 #   47.306 CPUs utilized            ( +-  0.10% )

             21.0004 +- 0.0265 seconds time elapsed  ( +-  0.13% )

That's a +45.8% speedup in elapsed time, and a +41.4% improvement in 
cpu-clock utilization.

I'm wondering whether your system has some sort of bottleneck?


One thing I do though when running benchmarks is to switch the cpufreq 
governor to 'performance', via something like:

   NR_CPUS=$(nproc --all)

   curr=$(cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor)
   next=performance

   echo "# setting all $NR_CPUS CPUs from '"$curr"' to the '"$next"' governor"

   for ((cpu=0; cpu<$NR_CPUS; cpu++)); do
     G=/sys/devices/system/cpu/cpu$cpu/cpufreq/scaling_governor
     [ -f $G ] && echo $next > $G
   done

This minimizes the amount of noise across iterations and makes the results 
more dependable:

             30.6258 +- 0.0337 seconds time elapsed  ( +-  0.11% )
             21.0004 +- 0.0265 seconds time elapsed  ( +-  0.13% )

> > With the fast-headers kernel that's down to ~36,000 lines of code, 
> > almost a factor of 3 reduction:
> > 
> >   # fast-headers-v1:
> >   kepler:~/mingo.tip.git> wc -l kernel/pid.i
> >   35941 kernel/pid.i
> 
> Coming from someone who often has to reduce a preprocessed kernel source 
> file with creduce/cvise to report compiler bugs, this will be a very 
> welcomed change, as those tools will have to do less work, and I can get 
> my reports done faster.

That's nice, didn't think of that side effect.

Could you perhaps measure this too, to see how much of a benefit it is?

> ########################################################################
> 
> I took the series for a spin with clang and GCC on arm64 and x86_64 and
> I found a few warnings/errors.

Thank you!

> 1. Position of certain attributes
> 
> In some commits, you move the cacheline_aligned attributes from after
> the closing brace on structures to before the struct keyword, which
> causes clang to warn (and error with CONFIG_WERROR):
> 
> In file included from arch/arm64/kernel/asm-offsets.c:9:
> In file included from arch/arm64/kernel/../../../kernel/sched/per_task_area_struct.h:33:
> In file included from ./include/linux/perf_event_api.h:17:
> In file included from ./include/linux/perf_event_types.h:41:
> In file included from ./include/linux/ftrace.h:18:
> In file included from ./arch/arm64/include/asm/ftrace.h:53:
> In file included from ./include/linux/compat.h:11:
> ./include/linux/fs_types.h:997:1: error: attribute '__aligned__' is ignored, place it after "struct" to apply attribute to type declaration [-Werror,-Wignored-attributes]
> ____cacheline_aligned
> ^
> ./include/linux/cache.h:41:46: note: expanded from macro '____cacheline_aligned'
> #define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))

Yeah, so this is a *really* stupid warning from Clang.

Putting the attribute after 'struct' risks the hard to track down bugs when 
a <linux/cache.h> inclusion is missing, which scenario I pointed out in 
this commit:

    headers/deps: dcache: Move the ____cacheline_aligned attribute to the head of the definition
    
    When changing <linux/dcache.h> I removed the <linux/spinlock_api.h> header,
    which caused a couple of hundred of mysterious, somewhat obscure link time errors:
    
      ld: net/sctp/tsnmap.o:(.bss+0x0): multiple definition of `____cacheline_aligned_in_smp'; init/do_mounts_rd.o:(.bss+0x0): first defined here
      ld: net/sctp/tsnmap.o:(.bss+0x40): multiple definition of `____cacheline_aligned'; init/do_mounts_rd.o:(.bss+0x40): first defined here
      ld: net/sctp/debug.o:(.bss+0x0): multiple definition of `____cacheline_aligned_in_smp'; init/do_mounts_rd.o:(.bss+0x0): first defined here
      ld: net/sctp/debug.o:(.bss+0x40): multiple definition of `____cacheline_aligned'; init/do_mounts_rd.o:(.bss+0x40): first defined here
    
    After a bit of head-scratching, what happened is that 'struct dentry_operations'
    has the ____cacheline_aligned attribute at the tail of the type definition -
    which turned into a local variable definition when <linux/cache.h> was not
    included - which <linux/spinlock_api.h> includes into <linux/dcache.h> indirectly.
    
    There were no compile time errors, only link time errors.
    
    Move the attribute to the head of the definition, in which case
    a missing <linux/cache.h> inclusion creates an immediate build failure:
    
      In file included from ./include/linux/fs.h:9,
                       from ./include/linux/fsverity.h:14,
                       from fs/verity/fsverity_private.h:18,
                       from fs/verity/read_metadata.c:8:
      ./include/linux/dcache.h:132:22: error: expected ‘;’ before ‘struct’
        132 | ____cacheline_aligned
            |                      ^
            |                      ;
        133 | struct dentry_operations {
            | ~~~~~~
    
    No change in functionality.
    
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

Can this Clang warning be disabled?

> 2. Error with CONFIG_SHADOW_CALL_STACK

So this feature depends on Clang:

 # Supported by clang >= 7.0
 config CC_HAVE_SHADOW_CALL_STACK
         def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)

No way to activate it under my GCC cross-build toolchain, right?

But ... I hacked the build mode on with GCC using this patch:

From: Ingo Molnar <mingo@kernel.org>
Date: Tue, 4 Jan 2022 11:26:09 +0100
Subject: [PATCH] DO NOT MERGE: Enable SHADOW_CALL_STACK on GCC builds, for build testing

NOT-Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 Makefile           | 2 +-
 arch/Kconfig       | 2 +-
 arch/arm64/Kconfig | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 16d7f83ac368..bbab462e7509 100644
--- a/Makefile
+++ b/Makefile
@@ -888,7 +888,7 @@ LDFLAGS_vmlinux += --gc-sections
 endif
 
 ifdef CONFIG_SHADOW_CALL_STACK
-CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
+CC_FLAGS_SCS	:=
 KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
 export CC_FLAGS_SCS
 endif
diff --git a/arch/Kconfig b/arch/Kconfig
index 4e56f66fdbcf..2103d9da4fe1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -605,7 +605,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
 
 config SHADOW_CALL_STACK
 	bool "Clang Shadow Call Stack"
-	depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
+	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
 	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
 	help
 	  This option enables Clang's Shadow Call Stack, which uses a
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index c4207cf9bb17..952f3e56e0a7 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1183,7 +1183,7 @@ config ARCH_HAS_FILTER_PGPROT
 
 # Supported by clang >= 7.0
 config CC_HAVE_SHADOW_CALL_STACK
-	def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
+	def_bool y
 
 config PARAVIRT
 	bool "Enable paravirtualization code"


And was able to trigger at least some of the build errors you saw:

  In file included from kernel/scs.c:15:
  ./include/linux/scs.h: In function 'scs_task_reset':
  ./include/linux/scs.h:26:34: error: implicit declaration of function 'task_thread_info' [-Werror=implicit-function-declaration]

This is fixed with:

diff --git a/kernel/scs.c b/kernel/scs.c
index ca9e707049cb..719ab53adc8a 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -5,6 +5,7 @@
  * Copyright (C) 2019 Google LLC
  */
 
+#include <linux/sched/thread_info_api.h>
 #include <linux/sched.h>
 #include <linux/mm_page_address.h>
 #include <linux/mm_api.h>


Then there's the build failure in init/main.c:

> It looks like on mainline, init_shadow_call_stack is in defined and used 
> in init/init_task.c but now, it is used in init/main.c, with no
> declaration to allow the compiler to find the definition. I guess moving
> init_shadow_call_stack out of init/init_task.c to somewhere more common
> would fix this but it depends on SCS_SIZE, which is defined in
> include/linux/scs.h, and as soon as I tried to include that in another
> file, the build broke further... Any ideas you have would be appreciated
> :) for benchmarking purposes, I just disabled CONFIG_SHADOW_CALL_STACK.

So I see:

In file included from ./include/linux/thread_info.h:63,
                 from ./arch/arm64/include/asm/smp.h:32,
                 from ./include/linux/smp_api.h:15,
                 from ./include/linux/percpu.h:6,
                 from ./include/linux/softirq.h:8,
                 from init/main.c:17:
init/main.c: In function 'init_per_task_early':
./arch/arm64/include/asm/thread_info.h:113:27: error: 'init_shadow_call_stack' undeclared (first use in this function)
  113 |         .scs_base       = init_shadow_call_stack,                       \
      |                           ^~~~~~~~~~~~~~~~~~~~~~

This looks pretty straightforward, does this patch solve it?

 include/linux/scs.h | 3 +++
 init/main.c         | 1 +
 2 files changed, 4 insertions(+)

diff --git a/include/linux/scs.h b/include/linux/scs.h
index 18122d9e17ff..863932a9347a 100644
--- a/include/linux/scs.h
+++ b/include/linux/scs.h
@@ -8,6 +8,7 @@
 #ifndef _LINUX_SCS_H
 #define _LINUX_SCS_H
 
+#include <linux/sched/thread_info_api.h>
 #include <linux/gfp.h>
 #include <linux/poison.h>
 #include <linux/sched.h>
@@ -25,6 +26,8 @@
 #define task_scs(tsk)		(task_thread_info(tsk)->scs_base)
 #define task_scs_sp(tsk)	(task_thread_info(tsk)->scs_sp)
 
+extern unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)];
+
 void *scs_alloc(int node);
 void scs_free(void *s);
 void scs_init(void);
diff --git a/init/main.c b/init/main.c
index c9eb3ecbe18c..74ccad445009 100644
--- a/init/main.c
+++ b/init/main.c
@@ -12,6 +12,7 @@
 
 #define DEBUG		/* Enable initcall_debug */
 
+#include <linux/scs.h>
 #include <linux/workqueue_api.h>
 #include <linux/sysctl.h>
 #include <linux/softirq.h>

I've applied these fixes, with that CONFIG_SHADOW_CALL_STACK=y builds fine 
on ARM64 - but I performed no runtime testing.

I've backmerged this into:

    headers/deps: per_task, arm64, x86: Convert task_struct::thread to a per_task() field

where this bug originated from.

I.e. I think the bug was simply to make main.c aware of the array, now that 
the INIT_THREAD initialization is done there.

We could move over the init_shadow_call_stack[] array there and make it 
static to begin with? I don't think anything truly relies on it being a 
global symbol.

> 3. Nested function in arch/x86/kernel/asm-offsets.c

> diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> index ff3f8ed5d0a2..a6d56f4697cd 100644
> --- a/arch/x86/kernel/asm-offsets.c
> +++ b/arch/x86/kernel/asm-offsets.c
> @@ -35,10 +35,10 @@
>  # include "asm-offsets_64.c"
>  #endif
> 
> -static void __used common(void)
> -{
>  #include "../../../kernel/sched/per_task_area_struct_defs.h"
> 
> +static void __used common(void)
> +{
>         BLANK();
>         DEFINE(TASK_threadsp, offsetof(struct task_struct, per_task_area) +
>                               offsetof(struct task_struct_per_task, thread) +

Ha, that code is bogus, it's a merge bug of mine. Super interesting that 
GCC still managed to include the header ...

I've applied your fix.

> 4. Build error in kernel/gcov/clang.c

> 8 errors generated.
> 
> I resolved this with:
> 
> diff --git a/kernel/gcov/clang.c b/kernel/gcov/clang.c
> index 6ee385f6ad47..29f0899ba209 100644
> --- a/kernel/gcov/clang.c
> +++ b/kernel/gcov/clang.c
> @@ -52,6 +52,7 @@
>  #include <linux/ratelimit.h>
>  #include <linux/slab.h>
>  #include <linux/mm.h>
> +#include <linux/string.h>
>  #include "gcov.h"

Thank you - applied!

>  typedef void (*llvm_gcov_callback)(void);
> 
> 
> 5. BPF errors
> 
> With Arch Linux's config (https://github.com/archlinux/svntogit-packages/raw/packages/linux/trunk/config),
> I see the following errors:
> 
> kernel/bpf/preload/iterators/iterators.c:3:10: fatal error: 'linux/sched/signal.h' file not found
> #include <linux/sched/signal.h>
>          ^~~~~~~~~~~~~~~~~~~~~~
> 1 error generated.
> 
> kernel/bpf/sysfs_btf.c:21:2: error: implicitly declaring library function 'memcpy' with type 'void *(void *, const void *, unsigned long)' [-Werror,-Wimplicit-function-declaration]
>         memcpy(buf, __start_BTF + off, len);
>         ^
> kernel/bpf/sysfs_btf.c:21:2: note: include the header <string.h> or explicitly provide a declaration for 'memcpy'
> 1 error generated.
> 
> The second error is obviously fixed by just including string.h as above.

Applied.

> I am not sure what is wrong with the first one; the includes all appear
> to be userland headers, rather than kernel ones, so maybe an -I flag is
> not present that should be? To work around it, I disabled
> CONFIG_BPF_PRELOAD.

Yeah, this should be fixed by simply removing the two stray dependencies 
that found their way into this user-space code:

 kernel/bpf/preload/iterators/iterators.bpf.c | 1 -
 kernel/bpf/preload/iterators/iterators.c     | 1 -
 2 files changed, 2 deletions(-)

diff --git a/kernel/bpf/preload/iterators/iterators.bpf.c b/kernel/bpf/preload/iterators/iterators.bpf.c
index 41ae00edeecf..03af863314ea 100644
--- a/kernel/bpf/preload/iterators/iterators.bpf.c
+++ b/kernel/bpf/preload/iterators/iterators.bpf.c
@@ -1,6 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2020 Facebook */
-#include <linux/seq_file.h>
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_core_read.h>
diff --git a/kernel/bpf/preload/iterators/iterators.c b/kernel/bpf/preload/iterators/iterators.c
index d702cbf7ddaf..5d872a705470 100644
--- a/kernel/bpf/preload/iterators/iterators.c
+++ b/kernel/bpf/preload/iterators/iterators.c
@@ -1,6 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2020 Facebook */
-#include <linux/sched/signal.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>


> 6. resolve_btfids warning
> 
> After working around the above errors, with either GCC or clang, I see
> the following warnings with Arch Linux's configuration:
> 
> WARN: multiple IDs found for 'task_struct': 103, 23549 - using 103
> WARN: multiple IDs found for 'path': 1166, 23551 - using 1166
> WARN: multiple IDs found for 'inode': 997, 23561 - using 997
> WARN: multiple IDs found for 'file': 714, 23566 - using 714
> WARN: multiple IDs found for 'seq_file': 1120, 23673 - using 1120
> 
> Which appears to come from symbols_resolve() in
> tools/bpf/resolve_btfids/main.c.

Hm, is this perhaps related to CONFIG_KALLSYMS_FAST=y? If yes then turning 
it off might help.

I don't really know this area of BPF all that much, maybe someone else can 
see what the problem is? The error message is not self-explanatory.

> 
> ########################################################################
> 
> I am very excited to see where this goes, it is a herculean effort but I
> think it will be worth it in the long run. Let me know if there is any
> more information or input that I can provide, cheers!

Your testing & patch sending efforts are much appreciated!! You'd help me 
most by continuing on the same path with new fast-headers releases as well, 
whenever you find the time. :-)

BTW., you can always pick up my latest Work-In-Progress branch from:

   git://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git sched/headers

The 'master' branch will carry the release.

The sched/headers branch is already rebased to -rc8 and has some other 
changes as well. It should normally work, with less testing than the main 
releasees, but will at times have fixes at the tail waiting to be 
backmerged in a bisect-friendly way.

Thanks,

	Ingo