[PATCH] x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86

[Ingo Molnar <mingo@kernel.org>]

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

> 
> * Dave Hansen <dave@sr71.net> wrote:
> 
> > >>  #endif /* _ASM_X86_FPU_H */
> > >> diff -puN arch/x86/kernel/process.c~dynamically-allocate-struct-fpu arch/x86/kernel/process.c
> > >> --- a/arch/x86/kernel/process.c~dynamically-allocate-struct-fpu 2015-07-16 10:50:42.360571875 -0700
> > >> +++ b/arch/x86/kernel/process.c 2015-07-16 12:00:59.204808551 -0700
> > >> @@ -81,7 +81,7 @@ EXPORT_SYMBOL_GPL(idle_notifier_unregist
> > >>   */
> > >>  int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> > >>  {
> > >> -       *dst = *src;
> > >> +       memcpy(dst, src, arch_task_struct_size());
> > > 
> > > This is actually vaguely performance-critical, which makes me thing
> > > that using some kind of inline or other real way (config macro, ifdef,
> > > etc) to detect whether there's an arch override would be better than a
> > > weak function.
> > 
> > Fair enough.  I'll send out another version in a bit if there are no more 
> > comments.
> 
> Beyond making it a build time switch for other architectures, I'd also suggest 
> introducing a __read_mostly task_struct_size variable on x86, so that we can 
> write:
> 
> 	memcpy(dst, src, task_struct_size);

I.e. like the patch below, on top of your patch plus the bug.h patch I sent.

Only build tested.

Thanks,

	Ingo

======================>
>From ab5da866792e4297de2308b42121633419f6d06d Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Fri, 17 Jul 2015 11:27:36 +0200
Subject: [PATCH] x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86

Don't burden architectures without dynamic task_struct sizing with the overhead
of dynamic sizing.

Also optimize the x86 code a bit by caching task_struct_size.

Cc: Dave Hansen <dave@sr71.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@alien8.de
Cc: luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/Kconfig               |  4 ++++
 arch/x86/Kconfig           |  1 +
 arch/x86/kernel/fpu/init.c |  9 ++++++---
 arch/x86/kernel/process.c  |  2 +-
 fs/proc/kcore.c            |  4 ++--
 include/linux/sched.h      |  4 +++-
 kernel/fork.c              | 21 ++++++++-------------
 7 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index bec6666a3cc4..dd6867885c73 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -221,6 +221,10 @@ config ARCH_TASK_STRUCT_ALLOCATOR
 config ARCH_THREAD_INFO_ALLOCATOR
 	bool
 
+# Select if arch wants to size task_struct dynamically
+config ARCH_WANTS_DYNAMIC_TASK_STRUCT
+	bool
+
 config HAVE_REGS_AND_STACK_ACCESS_API
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b6b03b716302..1e1d0e8ac9b6 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -41,6 +41,7 @@ config X86
 	select ARCH_USE_CMPXCHG_LOCKREF		if X86_64
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_USE_QUEUED_SPINLOCKS
+	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_WANT_IPC_PARSE_VERSION	if X86_32
 	select ARCH_WANT_OPTIONAL_GPIOLIB
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 36de0ebdb957..5401243430ee 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -4,6 +4,8 @@
 #include <asm/fpu/internal.h>
 #include <asm/tlbflush.h>
 
+#include <linux/sched.h>
+
 /*
  * Initialize the TS bit in CR0 according to the style of context-switches
  * we are using:
@@ -137,9 +139,9 @@ unsigned int xstate_size;
 EXPORT_SYMBOL_GPL(xstate_size);
 
 /*
- * We append the 'struct fpu' to the task_struct.
+ * We append the 'struct fpu' to the task_struct:
  */
-int arch_task_struct_size(void)
+static void __init fpu__init_task_struct_size(void)
 {
 	int task_size = sizeof(struct task_struct);
 
@@ -159,7 +161,7 @@ int arch_task_struct_size(void)
 	CHECK_MEMBER_AT_END_OF(struct fpu, state);
 	CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
 
-	return task_size;
+	task_struct_size = task_size;
 }
 
 /*
@@ -313,6 +315,7 @@ void __init fpu__init_system(struct cpuinfo_x86 *c)
 	fpu__init_system_generic();
 	fpu__init_system_xstate_size_legacy();
 	fpu__init_system_xstate();
+	fpu__init_task_struct_size();
 
 	fpu__init_system_ctx_switch();
 }
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 975420eac105..ae6a315cfa73 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -81,7 +81,7 @@ EXPORT_SYMBOL_GPL(idle_notifier_unregister);
  */
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
-	memcpy(dst, src, arch_task_struct_size());
+	memcpy(dst, src, task_struct_size);
 
 	return fpu__copy(&dst->thread.fpu, &src->thread.fpu);
 }
diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index a0fe99485687..1cfa1127c9dd 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -92,7 +92,7 @@ static size_t get_kcore_size(int *nphdr, size_t *elf_buflen)
 			     roundup(sizeof(CORE_STR), 4)) +
 			roundup(sizeof(struct elf_prstatus), 4) +
 			roundup(sizeof(struct elf_prpsinfo), 4) +
-			roundup(arch_task_struct_size(), 4);
+			roundup(task_struct_size, 4);
 	*elf_buflen = PAGE_ALIGN(*elf_buflen);
 	return size + *elf_buflen;
 }
@@ -415,7 +415,7 @@ static void elf_kcore_store_hdr(char *bufp, int nphdr, int dataoff)
 	/* set up the task structure */
 	notes[2].name	= CORE_STR;
 	notes[2].type	= NT_TASKSTRUCT;
-	notes[2].datasz	= arch_task_struct_size();
+	notes[2].datasz	= task_struct_size;
 	notes[2].data	= current;
 
 	nhdr->p_filesz	+= notesize(&notes[2]);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e43a41d892b6..d0504bb9f12a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1786,7 +1786,9 @@ struct task_struct {
  */
 };
 
-extern int arch_task_struct_size(void);
+#ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
+extern int task_struct_size __read_mostly;
+#endif
 
 /* Future-safe accessor for struct task_struct's cpus_allowed. */
 #define tsk_cpus_allowed(tsk) (&(tsk)->cpus_allowed)
diff --git a/kernel/fork.c b/kernel/fork.c
index 9f84b1aa3dbf..cfbe24ddf3eb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -287,14 +287,15 @@ static void set_max_threads(unsigned int max_threads_suggested)
 	max_threads = clamp_t(u64, threads, MIN_THREADS, MAX_THREADS);
 }
 
-#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \
-	BUILD_BUG_ON(sizeof(TYPE) != offsetofend(TYPE, MEMBER))
+#ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
+/* Initialized by the architecture: */
+int task_struct_size __read_mostly;
+#else
+/* Static build time size: */
+static const int task_struct_size = sizeof(struct task_struct);
+#endif
 
-/*
- * This function can be overridden by the architecture to support dynamic sizing
- * of the task_struct:
- */
-int __weak arch_task_struct_size(void)
+void __init fork_init(void)
 {
 	/*
 	 * Build-time checks for structure layout constraints:
@@ -307,12 +308,6 @@ int __weak arch_task_struct_size(void)
 	 */
 	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
 
-	return sizeof(struct task_struct);
-}
-
-void __init fork_init(void)
-{
-	int task_struct_size = arch_task_struct_size();
 #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
 #ifndef ARCH_MIN_TASKALIGN
 #define ARCH_MIN_TASKALIGN	L1_CACHE_BYTES