[PATCH 12/20] ARC: __switch_to: move ksp to thread_info from thread_struct

Vineet Gupta
2023-08-15T00:49:28Z

task's arch specific bits are carried in 2 places
- embedded thread_struct in task_struct
- associated thread_info (hoisted in task's stack page) and
syntactically: (thread_info *)(task_struct->stack)

ksp (dynamic kernel stack top) currently lives in thread_struct but
given its deep location in task struct likely to cache miss when
accessed from __switch_to(). Moving it to thread_info would be more
efficient given proximity to frequently accessed items such as
preempt_count thus very likely to be in cache, specially in schedular
code.

Note however that currently tsk.thread.ksp takes 1 memory access (off
of tsk pointer) while new code tsk->stack.ksp would take 2, but likely
to be in cache. Moreover if task is current the 2nd reference can be
elided and instead derived from SP as (SP & ~(THREAD_SIZE - 1))

All of this also makes __switch_to() code simpler and we can see the 2
ways of retirving ksp (descrobed above) in new code.

Signed-off-by: Vineet Gupta <vgupta@kernel.org>
---
arch/arc/include/asm/processor.h | 3 +--
arch/arc/include/asm/thread_info.h | 10 +++++-----
arch/arc/kernel/asm-offsets.c | 2 +-
arch/arc/kernel/ctx_sw_asm.S | 22 ++++++++++------------
arch/arc/kernel/process.c | 6 +++---
5 files changed, 20 insertions(+), 23 deletions(-)

diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
index 82ddc929d6e7..d606658e2fe7 100644
--- a/arch/arc/include/asm/processor.h
+++ b/arch/arc/include/asm/processor.h
@@ -22,7 +22,6 @@
* struct thread_info
*/
struct thread_struct {
- unsigned long ksp; /* kernel mode stack pointer */
unsigned long callee_reg; /* pointer to callee regs */
unsigned long fault_address; /* dbls as brkpt holder as well */
#ifdef CONFIG_ARC_DSP_SAVE_RESTORE_REGS
@@ -54,7 +53,7 @@ struct task_struct;
* Where about of Task's sp, fp, blink when it was last seen in kernel mode.
* Look in process.c for details of kernel stack layout
*/
-#define TSK_K_ESP(tsk) (tsk->thread.ksp)
+#define TSK_K_ESP(tsk) (task_thread_info(tsk)->ksp)

#define TSK_K_REG(tsk, off) (*((unsigned long *)(TSK_K_ESP(tsk) + \
sizeof(struct callee_regs) + off)))
diff --git a/arch/arc/include/asm/thread_info.h b/arch/arc/include/asm/thread_info.h
index 6ba7fe417095..4c530cf131f3 100644
--- a/arch/arc/include/asm/thread_info.h
+++ b/arch/arc/include/asm/thread_info.h
@@ -37,16 +37,16 @@
*/
struct thread_info {
unsigned long flags; /* low level flags */
+ unsigned long ksp; /* kernel mode stack top in __switch_to */
int preempt_count; /* 0 => preemptable, <0 => BUG */
- struct task_struct *task; /* main task structure */
- __u32 cpu; /* current CPU */
+ int cpu; /* current CPU */
unsigned long thr_ptr; /* TLS ptr */
+ struct task_struct *task; /* main task structure */
};

/*
- * macros/functions for gaining access to the thread information structure
- *
- * preempt_count needs to be 1 initially, until the scheduler is functional.
+ * initilaize thread_info for any @tsk
+ * - this is not related to init_task per se
*/
#define INIT_THREAD_INFO(tsk) \
{ \
diff --git a/arch/arc/kernel/asm-offsets.c b/arch/arc/kernel/asm-offsets.c
index 37324fd9a72f..5b47d09b925e 100644
--- a/arch/arc/kernel/asm-offsets.c
+++ b/arch/arc/kernel/asm-offsets.c
@@ -20,13 +20,13 @@ int main(void)

BLANK();

- DEFINE(THREAD_KSP, offsetof(struct thread_struct, ksp));
DEFINE(THREAD_CALLEE_REG, offsetof(struct thread_struct, callee_reg));
DEFINE(THREAD_FAULT_ADDR,
offsetof(struct thread_struct, fault_address));

BLANK();

+ DEFINE(THREAD_INFO_KSP, offsetof(struct thread_info, ksp));
DEFINE(THREAD_INFO_FLAGS, offsetof(struct thread_info, flags));
DEFINE(THREAD_INFO_PREEMPT_COUNT,
offsetof(struct thread_info, preempt_count));
diff --git a/arch/arc/kernel/ctx_sw_asm.S b/arch/arc/kernel/ctx_sw_asm.S
index 59d779004e64..48e1f21976ed 100644
--- a/arch/arc/kernel/ctx_sw_asm.S
+++ b/arch/arc/kernel/ctx_sw_asm.S
@@ -11,8 +11,6 @@
#include <asm/entry.h> /* For the SAVE_* macros */
#include <asm/asm-offsets.h>

-#define KSP_WORD_OFF ((TASK_THREAD + THREAD_KSP) / 4)
-
; IN
; - r0: prev task (also current)
; - r1: next task
@@ -37,19 +35,19 @@ ENTRY_CFI(__switch_to)
/* kernel mode callee regs of @prev */
SAVE_CALLEE_SAVED_KERNEL

- /* save final SP to @prev->thread.ksp */
-#if KSP_WORD_OFF <= 255
- st.as sp, [r0, KSP_WORD_OFF]
-#else
- /* Workaround for NR_CPUS=4k as ST.as can only take s9 offset */
- add2 r10, r0, KSP_WORD_OFF
- st sp, [r10]
-#endif
+ /*
+ * save final SP to @prev->thread_info.ksp
+ * @prev is "current" so thread_info derived from SP
+ */
+ GET_CURR_THR_INFO_FROM_SP r10
+ st sp, [r10, THREAD_INFO_KSP]
+
/* update @next in _current_task[] and GP register caching it */
SET_CURR_TASK_ON_CPU r1, r10

- /* load SP from @next->thread.ksp */
- ld.as sp, [r1, KSP_WORD_OFF]
+ /* load SP from @next->thread_info.ksp */
+ ld r10, [r1, TASK_THREAD_INFO]
+ ld sp, [r10, THREAD_INFO_KSP]

/* restore callee regs, stack frame regs of @next */
RESTORE_CALLEE_SAVED_KERNEL
diff --git a/arch/arc/kernel/process.c b/arch/arc/kernel/process.c
index 96f591508142..186ceab661eb 100644
--- a/arch/arc/kernel/process.c
+++ b/arch/arc/kernel/process.c
@@ -141,7 +141,7 @@ asmlinkage void ret_from_fork(void);
* | unused |
* | |
* ------------------
- * | r25 | <==== top of Stack (thread.ksp)
+ * | r25 | <==== top of Stack (thread_info.ksp)
* ~ ~
* | --to-- | (CALLEE Regs of kernel mode)
* | r13 |
@@ -181,14 +181,14 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
c_callee = ((struct callee_regs *)childksp) - 1;

/*
- * __switch_to() uses thread.ksp to start unwinding stack
+ * __switch_to() uses thread_info.ksp to start unwinding stack
* For kernel threads we don't need to create callee regs, the
* stack layout nevertheless needs to remain the same.
* Also, since __switch_to anyways unwinds callee regs, we use
* this to populate kernel thread entry-pt/args into callee regs,
* so that ret_from_kernel_thread() becomes simpler.
*/
- p->thread.ksp = (unsigned long)c_callee; /* THREAD_KSP */
+ task_thread_info(p)->ksp = (unsigned long)c_callee; /* THREAD_INFO_KSP */

/* __switch_to expects FP(0), BLINK(return addr) at top */
childksp[0] = 0; /* fp */
--
2.34.1