From f94edacf998516ac9d849f7bc6949a703977a7f3 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 17 Feb 2012 21:48:54 -0800 Subject: i387: move TS_USEDFPU flag from thread_info to task_struct This moves the bit that indicates whether a thread has ownership of the FPU from the TS_USEDFPU bit in thread_info->status to a word of its own (called 'has_fpu') in task_struct->thread.has_fpu. This fixes two independent bugs at the same time: - changing 'thread_info->status' from the scheduler causes nasty problems for the other users of that variable, since it is defined to be thread-synchronous (that's what the "TS_" part of the naming was supposed to indicate). So perfectly valid code could (and did) do ti->status |= TS_RESTORE_SIGMASK; and the compiler was free to do that as separate load, or and store instructions. Which can cause problems with preemption, since a task switch could happen in between, and change the TS_USEDFPU bit. The change to TS_USEDFPU would be overwritten by the final store. In practice, this seldom happened, though, because the 'status' field was seldom used more than once, so gcc would generally tend to generate code that used a read-modify-write instruction and thus happened to avoid this problem - RMW instructions are naturally low fat and preemption-safe. - On x86-32, the current_thread_info() pointer would, during interrupts and softirqs, point to a *copy* of the real thread_info, because x86-32 uses %esp to calculate the thread_info address, and thus the separate irq (and softirq) stacks would cause these kinds of odd thread_info copy aliases. This is normally not a problem, since interrupts aren't supposed to look at thread information anyway (what thread is running at interrupt time really isn't very well-defined), but it confused the heck out of irq_fpu_usable() and the code that tried to squirrel away the FPU state. (It also caused untold confusion for us poor kernel developers). It also turns out that using 'task_struct' is actually much more natural for most of the call sites that care about the FPU state, since they tend to work with the task struct for other reasons anyway (ie scheduling). And the FPU data that we are going to save/restore is found there too. Thanks to Arjan Van De Ven for pointing us to the %esp issue. Cc: Arjan van de Ven Reported-and-tested-by: Raphael Prevost Acked-and-tested-by: Suresh Siddha Tested-by: Peter Anvin Signed-off-by: Linus Torvalds --- arch/x86/include/asm/processor.h | 1 + 1 file changed, 1 insertion(+) (limited to 'arch/x86/include/asm/processor.h') diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index aa9088c26931..f7c89e231c6c 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -454,6 +454,7 @@ struct thread_struct { unsigned long trap_no; unsigned long error_code; /* floating point and extended processor state */ + unsigned long has_fpu; struct fpu fpu; #ifdef CONFIG_X86_32 /* Virtual 86 mode info */ -- cgit v1.2.3 From 7e16838d94b566a17b65231073d179bc04d590c8 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 19 Feb 2012 13:27:00 -0800 Subject: i387: support lazy restore of FPU state This makes us recognize when we try to restore FPU state that matches what we already have in the FPU on this CPU, and avoids the restore entirely if so. To do this, we add two new data fields: - a percpu 'fpu_owner_task' variable that gets written any time we update the "has_fpu" field, and thus acts as a kind of back-pointer to the task that owns the CPU. The exception is when we save the FPU state as part of a context switch - if the save can keep the FPU state around, we leave the 'fpu_owner_task' variable pointing at the task whose FP state still remains on the CPU. - a per-thread 'last_cpu' field, that indicates which CPU that thread used its FPU on last. We update this on every context switch (writing an invalid CPU number if the last context switch didn't leave the FPU in a lazily usable state), so we know that *that* thread has done nothing else with the FPU since. These two fields together can be used when next switching back to the task to see if the CPU still matches: if 'fpu_owner_task' matches the task we are switching to, we know that no other task (or kernel FPU usage) touched the FPU on this CPU in the meantime, and if the current CPU number matches the 'last_cpu' field, we know that this thread did no other FP work on any other CPU, so the FPU state on the CPU must match what was saved on last context switch. In that case, we can avoid the 'f[x]rstor' entirely, and just clear the CR0.TS bit. Signed-off-by: Linus Torvalds --- arch/x86/include/asm/i387.h | 35 +++++++++++++++++++++++------------ arch/x86/include/asm/processor.h | 3 ++- arch/x86/kernel/cpu/common.c | 2 ++ arch/x86/kernel/process_32.c | 2 +- arch/x86/kernel/process_64.c | 2 +- 5 files changed, 29 insertions(+), 15 deletions(-) (limited to 'arch/x86/include/asm/processor.h') diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h index 74c607b37e87..247904945d3f 100644 --- a/arch/x86/include/asm/i387.h +++ b/arch/x86/include/asm/i387.h @@ -32,6 +32,8 @@ extern int init_fpu(struct task_struct *child); extern void math_state_restore(void); extern int dump_fpu(struct pt_regs *, struct user_i387_struct *); +DECLARE_PER_CPU(struct task_struct *, fpu_owner_task); + extern user_regset_active_fn fpregs_active, xfpregs_active; extern user_regset_get_fn fpregs_get, xfpregs_get, fpregs_soft_get, xstateregs_get; @@ -276,7 +278,7 @@ static inline int restore_fpu_checking(struct task_struct *tsk) "emms\n\t" /* clear stack tags */ "fildl %P[addr]", /* set F?P to defined value */ X86_FEATURE_FXSAVE_LEAK, - [addr] "m" (tsk->thread.has_fpu)); + [addr] "m" (tsk->thread.fpu.has_fpu)); return fpu_restore_checking(&tsk->thread.fpu); } @@ -288,19 +290,21 @@ static inline int restore_fpu_checking(struct task_struct *tsk) */ static inline int __thread_has_fpu(struct task_struct *tsk) { - return tsk->thread.has_fpu; + return tsk->thread.fpu.has_fpu; } /* Must be paired with an 'stts' after! */ static inline void __thread_clear_has_fpu(struct task_struct *tsk) { - tsk->thread.has_fpu = 0; + tsk->thread.fpu.has_fpu = 0; + percpu_write(fpu_owner_task, NULL); } /* Must be paired with a 'clts' before! */ static inline void __thread_set_has_fpu(struct task_struct *tsk) { - tsk->thread.has_fpu = 1; + tsk->thread.fpu.has_fpu = 1; + percpu_write(fpu_owner_task, tsk); } /* @@ -345,18 +349,22 @@ typedef struct { int preload; } fpu_switch_t; * We don't do that yet, so "fpu_lazy_restore()" always returns * false, but some day.. */ -#define fpu_lazy_restore(tsk) (0) -#define fpu_lazy_state_intact(tsk) do { } while (0) +static inline int fpu_lazy_restore(struct task_struct *new, unsigned int cpu) +{ + return new == percpu_read_stable(fpu_owner_task) && + cpu == new->thread.fpu.last_cpu; +} -static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct task_struct *new) +static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct task_struct *new, int cpu) { fpu_switch_t fpu; fpu.preload = tsk_used_math(new) && new->fpu_counter > 5; if (__thread_has_fpu(old)) { - if (__save_init_fpu(old)) - fpu_lazy_state_intact(old); - __thread_clear_has_fpu(old); + if (!__save_init_fpu(old)) + cpu = ~0; + old->thread.fpu.last_cpu = cpu; + old->thread.fpu.has_fpu = 0; /* But leave fpu_owner_task! */ /* Don't change CR0.TS if we just switch! */ if (fpu.preload) { @@ -367,9 +375,10 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta stts(); } else { old->fpu_counter = 0; + old->thread.fpu.last_cpu = ~0; if (fpu.preload) { new->fpu_counter++; - if (fpu_lazy_restore(new)) + if (fpu_lazy_restore(new, cpu)) fpu.preload = 0; else prefetch(new->thread.fpu.state); @@ -463,8 +472,10 @@ static inline void kernel_fpu_begin(void) __save_init_fpu(me); __thread_clear_has_fpu(me); /* We do 'stts()' in kernel_fpu_end() */ - } else + } else { + percpu_write(fpu_owner_task, NULL); clts(); + } } static inline void kernel_fpu_end(void) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index f7c89e231c6c..58545c97d071 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -374,6 +374,8 @@ union thread_xstate { }; struct fpu { + unsigned int last_cpu; + unsigned int has_fpu; union thread_xstate *state; }; @@ -454,7 +456,6 @@ struct thread_struct { unsigned long trap_no; unsigned long error_code; /* floating point and extended processor state */ - unsigned long has_fpu; struct fpu fpu; #ifdef CONFIG_X86_32 /* Virtual 86 mode info */ diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index d43cad74f166..b667148dfad7 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1044,6 +1044,8 @@ DEFINE_PER_CPU(char *, irq_stack_ptr) = DEFINE_PER_CPU(unsigned int, irq_count) = -1; +DEFINE_PER_CPU(struct task_struct *, fpu_owner_task); + /* * Special IST stacks which the CPU switches to when it calls * an IST-marked descriptor entry. Up to 7 stacks (hardware diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c index bc32761bc27a..c08d1ff12b7c 100644 --- a/arch/x86/kernel/process_32.c +++ b/arch/x86/kernel/process_32.c @@ -304,7 +304,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) /* never put a printk in __switch_to... printk() calls wake_up*() indirectly */ - fpu = switch_fpu_prepare(prev_p, next_p); + fpu = switch_fpu_prepare(prev_p, next_p, cpu); /* * Reload esp0. diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 8ad880b3bc1c..cfa5c90c01db 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -389,7 +389,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) unsigned fsindex, gsindex; fpu_switch_t fpu; - fpu = switch_fpu_prepare(prev_p, next_p); + fpu = switch_fpu_prepare(prev_p, next_p, cpu); /* * Reload esp0, LDT and the page table pointer: -- cgit v1.2.3