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Message-ID: <20250503143830.GA8982@redhat.com>
Date: Sat, 3 May 2025 16:38:30 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
Dave Hansen <dave@...1.net>, Brian Gerst <brgerst@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
"Chang S . Bae" <chang.seok.bae@...el.com>
Subject: [PATCH tip/x86/fpu 1/6] x86/fpu: simplify the switch_fpu_prepare() +
switch_fpu_finish() logic
Now that switch_fpu_finish() doesn't load the FPU state, it makes more
sense to fold it into switch_fpu_prepare() renamed to switch_fpu(), and
more importantly, use the "prev_p" task as a target for TIF_NEED_FPU_LOAD.
It doesn't make any sense to delay set_tsk_thread_flag(TIF_NEED_FPU_LOAD)
until "prev_p" is scheduled again.
There is no worry about the very first context switch, fpu_clone() must
always set TIF_NEED_FPU_LOAD.
Also, shift the test_tsk_thread_flag(TIF_NEED_FPU_LOAD) from the callers
to switch_fpu().
Note that the "PF_KTHREAD | PF_USER_WORKER" check can be removed but
this deserves a separate patch which can change more functions, say,
kernel_fpu_begin_mask().
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
arch/x86/include/asm/fpu/sched.h | 34 +++++++++-----------------------
arch/x86/kernel/process_32.c | 5 +----
arch/x86/kernel/process_64.c | 5 +----
3 files changed, 11 insertions(+), 33 deletions(-)
diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
index 5fd12634bcc4..c060549c6c94 100644
--- a/arch/x86/include/asm/fpu/sched.h
+++ b/arch/x86/include/asm/fpu/sched.h
@@ -18,31 +18,25 @@ extern void fpu_flush_thread(void);
/*
* FPU state switching for scheduling.
*
- * This is a two-stage process:
+ * switch_fpu() saves the old state and sets TIF_NEED_FPU_LOAD if
+ * TIF_NEED_FPU_LOAD is not set. This is done within the context
+ * of the old process.
*
- * - switch_fpu_prepare() saves the old state.
- * This is done within the context of the old process.
- *
- * - switch_fpu_finish() sets TIF_NEED_FPU_LOAD; the floating point state
- * will get loaded on return to userspace, or when the kernel needs it.
- *
- * If TIF_NEED_FPU_LOAD is cleared then the CPU's FPU registers
- * are saved in the current thread's FPU register state.
- *
- * If TIF_NEED_FPU_LOAD is set then CPU's FPU registers may not
- * hold current()'s FPU registers. It is required to load the
+ * Once TIF_NEED_FPU_LOAD is set, it is required to load the
* registers before returning to userland or using the content
* otherwise.
*
* The FPU context is only stored/restored for a user task and
* PF_KTHREAD is used to distinguish between kernel and user threads.
*/
-static inline void switch_fpu_prepare(struct task_struct *old, int cpu)
+static inline void switch_fpu(struct task_struct *old, int cpu)
{
- if (cpu_feature_enabled(X86_FEATURE_FPU) &&
+ if (!test_tsk_thread_flag(old, TIF_NEED_FPU_LOAD) &&
+ cpu_feature_enabled(X86_FEATURE_FPU) &&
!(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
struct fpu *old_fpu = x86_task_fpu(old);
+ set_tsk_thread_flag(old, TIF_NEED_FPU_LOAD);
save_fpregs_to_fpstate(old_fpu);
/*
* The save operation preserved register state, so the
@@ -50,7 +44,7 @@ static inline void switch_fpu_prepare(struct task_struct *old, int cpu)
* current CPU number in @old_fpu, so the next return
* to user space can avoid the FPU register restore
* when is returns on the same CPU and still owns the
- * context.
+ * context. See fpregs_restore_userregs().
*/
old_fpu->last_cpu = cpu;
@@ -58,14 +52,4 @@ static inline void switch_fpu_prepare(struct task_struct *old, int cpu)
}
}
-/*
- * Delay loading of the complete FPU state until the return to userland.
- * PKRU is handled separately.
- */
-static inline void switch_fpu_finish(struct task_struct *new)
-{
- if (cpu_feature_enabled(X86_FEATURE_FPU))
- set_tsk_thread_flag(new, TIF_NEED_FPU_LOAD);
-}
-
#endif /* _ASM_X86_FPU_SCHED_H */
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4636ef359973..9bd4fa694da5 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -160,8 +160,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
- if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
- switch_fpu_prepare(prev_p, cpu);
+ switch_fpu(prev_p, cpu);
/*
* Save away %gs. No need to save %fs, as it was saved on the
@@ -208,8 +207,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
raw_cpu_write(current_task, next_p);
- switch_fpu_finish(next_p);
-
/* Load the Intel cache allocation PQR MSR. */
resctrl_sched_in(next_p);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 7196ca7048be..d55310d3133c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -616,8 +616,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) &&
this_cpu_read(hardirq_stack_inuse));
- if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
- switch_fpu_prepare(prev_p, cpu);
+ switch_fpu(prev_p, cpu);
/* We must save %fs and %gs before load_TLS() because
* %fs and %gs may be cleared by load_TLS().
@@ -671,8 +670,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
raw_cpu_write(current_task, next_p);
raw_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));
- switch_fpu_finish(next_p);
-
/* Reload sp0. */
update_task_stack(next_p);
--
2.25.1.362.g51ebf55
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