[PATCH] per_task: Remove the PER_TASK_BYTES hard-coded constant

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> There's one thing ugly about it, the fixed PER_TASK_BYTES limit, I plan 
> to make ->per_task_array[] the last field of task_struct, i.e. change it 
> to:
> 
>         u8                              per_task_area[];
> 
> This actually became possible through the fixing of the x86 FPU code in the 
> following fast-headers commit:
> 
>    4ae0f28bc1c8 headers/deps: x86/fpu: Make task_struct::thread constant size

So I implemented this approach - the patch below removes the PER_TASK_BYTES 
hard-coded limit.

( Didn't make it variable size via per_task_area[] though - we *do* know 
  its size after all at build time already, and known-size structures are 
  better in general than tail-variable-array solutions:

   - They work better with static checkers,
   - and we actually want the offsets into thread_info to be small on embedded platforms

  etc. )

Thanks,

	Ingo

============================>
From: Ingo Molnar <mingo@kernel.org>
Date: Tue, 4 Jan 2022 13:48:05 +0100
Subject: [PATCH] per_task: Remove the PER_TASK_BYTES hard-coded constant

- Also remove the unnecessary <linux/sched/per_task_types.h> header.

Not-Signed-off-by-yet: Ingo Molnar <mingo@kernel.org>
---
 include/linux/sched/per_task.h       | 3 ++-
 include/linux/sched/per_task_types.h | 7 -------
 kernel/sched/core.c                  | 4 ++++
 3 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched/per_task.h b/include/linux/sched/per_task.h
index e20837e82681..a10538713a26 100644
--- a/include/linux/sched/per_task.h
+++ b/include/linux/sched/per_task.h
@@ -37,7 +37,6 @@
  * A build-time check ensures that we haven't run out of available space.
  */
 
-#include <linux/sched/per_task_types.h>
 #include <linux/compiler.h>
 
 #ifndef __PER_TASK_GEN
@@ -61,4 +60,6 @@
 
 #define per_task_container_of(var, name)	container_of((void *)(var) - per_task_offset(name), struct task_struct, per_task_area[0])
 
+#define PER_TASK_BYTES				(per_task_offset(_end))
+
 #endif /* _LINUX_SCHED_PER_TASK_H */
diff --git a/include/linux/sched/per_task_types.h b/include/linux/sched/per_task_types.h
deleted file mode 100644
index 8af8c10f8dae..000000000000
--- a/include/linux/sched/per_task_types.h
+++ /dev/null
@@ -1,7 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _LINUX_SCHED_PER_TASK_TYPES_H
-#define _LINUX_SCHED_PER_TASK_TYPES_H
-
-#define PER_TASK_BYTES 8192
-
-#endif /* _LINUX_SCHED_PER_TASK_TYPES_H */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bc38b19f6398..fdb5b99ae6e0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -89,6 +89,8 @@
 #include "../../fs/io-wq.h"
 #include "../smpboot.h"
 
+#include "../../../kernel/sched/per_task_area_struct.h"
+
 DEFINE_PER_TASK(unsigned int,				flags);
 
 #ifdef CONFIG_THREAD_INFO_IN_TASK
@@ -9481,6 +9483,8 @@ void __init per_task_init(void)
 {
 	unsigned long per_task_bytes = per_task_offset(_end);
 
+	printk("per_task: sizeof(struct task_struct):          %ld bytes\n", sizeof(struct task_struct));
+	printk("per_task: sizeof(struct task_struct_per_task): %ld bytes\n", sizeof(struct task_struct_per_task));
 	printk("per_task: Using %ld per_task bytes, %ld bytes available\n", per_task_bytes, (long)PER_TASK_BYTES);
 
 	BUG_ON(per_task_offset(_end) > PER_TASK_BYTES);