[PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization

[PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

When both PREEMPT_RT and RANDOM_KMALLOC_CACHES are enabled, the slub allocator
runs into a build time failure:

In file included from <command-line>:
In function 'alloc_kmem_cache_cpus',
    inlined from 'do_kmem_cache_create' at mm/slub.c:6041:7:
include/linux/compiler_types.h:517:45: error: call to '__compiletime_assert_598' declared with attribute error: BUILD_BUG_ON failed: PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu)
  517 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                             ^
include/linux/compiler_types.h:498:25: note: in definition of macro '__compiletime_assert'
  498 |                         prefix ## suffix();                             \
      |                         ^~~~~~
include/linux/compiler_types.h:517:9: note: in expansion of macro '_compiletime_assert'
  517 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |         ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |         ^~~~~~~~~~~~~~~~
mm/slub.c:5133:9: note: in expansion of macro 'BUILD_BUG_ON'
 5133 |         BUILD_BUG_ON(PERCPU_DYNAMIC_EARLY_SIZE <
      |         ^~~~~~~~~~~~

The problem is the additional size overhead from local_lock in
struct kmem_cache_cpu.  Avoid this by preallocating a larger area.

Fixes: d8fccd9ca5f9 ("arm64: Allow to enable PREEMPT_RT.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410020326.iaZIteIx-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
There is a good chance that there is a better way to address this, this
version was the first I came up with and I verified that it fixes all of
the broken configs.

See https://pastebin.com/raw/tuPgfPzu for a .config from a failing
randconfig build on 6.12-rc1.
---
 include/linux/percpu.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index b6321fc49159..4083295da27f 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -41,7 +41,11 @@
 					 PCPU_MIN_ALLOC_SHIFT)
 
 #ifdef CONFIG_RANDOM_KMALLOC_CACHES
+#ifdef CONFIG_PREEMPT_RT
+#define PERCPU_DYNAMIC_SIZE_SHIFT      13
+#else
 #define PERCPU_DYNAMIC_SIZE_SHIFT      12
+#endif
 #else
 #define PERCPU_DYNAMIC_SIZE_SHIFT      10
 #endif
-- 
2.39.2

Re: [PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization

[Sebastian Andrzej Siewior]

On 2024-10-04 09:56:56 [+0000], Arnd Bergmann wrote:
> The problem is the additional size overhead from local_lock in
> struct kmem_cache_cpu.  Avoid this by preallocating a larger area.

The worst case would be enabling additionally MEMCG so NR_KMALLOC_TYPES
increases by one. And then we have:
PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu));

There is more to it than just RANDOM_KMALLOC_CACHES and PREEMPT_RT.
There is additionally CONFIG_LOCKDEP which increases the size of
local_lock_t further. Plus CONFIG_LOCK_STAT. The last one a kind of bad
in terms of required pad area. Then we have CONFIG_PAGE_SIZE_64KB set
which is the culprit. But 16K_PAGES also fail in this full blown case.

PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu));
4KiB   20 << 12           <     19           *   (12 + 1)         * 288
         80KiB            <           69.46875
16KiB  20 << 12           <     19           *   (14 + 1)         * 288
         80KiB            <           80.15625
64KiB  20 << 12           <     19           *   (16 + 1)         * 288
         80KiB            <           90.84375
128KiB 20 << 12           <     19           *   (17 + 1)         * 288
         80KiB            <           96.1875

Just disabling CONFIG_LOCK_STAT the 16KiB PAGE_SIZE case works again
(75.703125), the 64KiB still fails (85.796875).

> There is a good chance that there is a better way to address this, this
> version was the first I came up with and I verified that it fixes all of
> the broken configs.

How bad is it, to have PERCPU_DYNAMIC_SIZE_SHIFT unconditionally set to
13? If it is bad could we restrict it with LOCKDEP and PAGE_SIZE > 4KiB?

So maybe something like this:

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index b6321fc491598..52b5ea663b9f0 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -41,7 +41,11 @@
 					 PCPU_MIN_ALLOC_SHIFT)
 
 #ifdef CONFIG_RANDOM_KMALLOC_CACHES
-#define PERCPU_DYNAMIC_SIZE_SHIFT      12
+# if defined(CONFIG_LOCKDEP) && !defined(CONFIG_PAGE_SIZE_4KB)
+# define PERCPU_DYNAMIC_SIZE_SHIFT      13
+# else
+# define PERCPU_DYNAMIC_SIZE_SHIFT      12
+#endif /* LOCKDEP and PAGE_SIZE > 4KiB */
 #else
 #define PERCPU_DYNAMIC_SIZE_SHIFT      10
 #endif

Sebastian

Re: [PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization

[Arnd Bergmann]

On Mon, Oct 7, 2024, at 10:43, Sebastian Andrzej Siewior wrote:
> On 2024-10-04 09:56:56 [+0000], Arnd Bergmann wrote:
> How bad is it, to have PERCPU_DYNAMIC_SIZE_SHIFT unconditionally set to
> 13? If it is bad could we restrict it with LOCKDEP and PAGE_SIZE > 4KiB?
>
> So maybe something like this:
>
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index b6321fc491598..52b5ea663b9f0 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -41,7 +41,11 @@
>  					 PCPU_MIN_ALLOC_SHIFT)
> 
>  #ifdef CONFIG_RANDOM_KMALLOC_CACHES
> -#define PERCPU_DYNAMIC_SIZE_SHIFT      12
> +# if defined(CONFIG_LOCKDEP) && !defined(CONFIG_PAGE_SIZE_4KB)
> +# define PERCPU_DYNAMIC_SIZE_SHIFT      13
> +# else
> +# define PERCPU_DYNAMIC_SIZE_SHIFT      12
> +#endif /* LOCKDEP and PAGE_SIZE > 4KiB */
>  #else
>  #define PERCPU_DYNAMIC_SIZE_SHIFT      10
>  #endif

I think that's fine. If you have lockdep and large page sizes,
the percpu memory area is entirely lost in the noise of the
overhead you already get.

For your version:

Acked-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Arnd Bergmann <arnd@arndb.de>

Can you pick that up for rt fixes (if you already have a tree)
or send it to Andrew for the mm tree? Feel free to take my
changelog text.

     Arnd

Re: [PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization

[Sebastian Andrzej Siewior]

On 2024-10-07 10:59:36 [+0000], Arnd Bergmann wrote:
> 
> Can you pick that up for rt fixes (if you already have a tree)
> or send it to Andrew for the mm tree? Feel free to take my
> changelog text.

I make a patch send it to mm so akpm/ Vlastimil can pick it up.
Thanks Arnd.

>      Arnd

Sebastian