Re: [RFC PATCH 02/11] x86,fpu: replace fpu_switch_t with a thread flag

Oleg Nesterov
2015-01-13T15:25:14Z

Rik,

I can't review this series, I forgot almost everything I learned about
this code. The only thing I can recall is that it needs cleanups and
fixes ;) Just a couple of random questions.

On 01/11, riel@redhat.com wrote:
>
> +static inline void switch_fpu_prepare(struct task_struct *old, struct task_struct *new, int cpu)
> {
> - fpu_switch_t fpu;
> -
> /*
> * If the task has used the math, pre-load the FPU on xsave processors
> * or if the past 5 consecutive context-switches used math.
> */
> - fpu.preload = tsk_used_math(new) && (use_eager_fpu() ||
> + bool preload = tsk_used_math(new) && (use_eager_fpu() ||
> new->thread.fpu_counter > 5);
> if (__thread_has_fpu(old)) {
> if (!__save_init_fpu(old))
> @@ -433,8 +417,9 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
> old->thread.fpu.has_fpu = 0; /* But leave fpu_owner_task! */
>
> /* Don't change CR0.TS if we just switch! */
> - if (fpu.preload) {
> + if (preload) {
> new->thread.fpu_counter++;
> + set_thread_flag(TIF_LOAD_FPU);
> __thread_set_has_fpu(new);
> prefetch(new->thread.fpu.state);
> } else if (!use_eager_fpu())
> @@ -442,16 +427,19 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
> } else {
> old->thread.fpu_counter = 0;
> old->thread.fpu.last_cpu = ~0;
> - if (fpu.preload) {
> + if (preload) {
> new->thread.fpu_counter++;
> if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
> - fpu.preload = 0;
> - else
> + /* XXX: is this safe against ptrace??? */

Could you explain your concerns?

> + __thread_fpu_begin(new);

this looks strange/unnecessary, there is another unconditonal
__thread_fpu_begin(new) below.

OK, the next patch moves it to switch_fpu_finish(), so perhaps this change
should go into 3/11.

And I am not sure I understand set_thread_flag(TIF_LOAD_FPU). This is called
before __switch_to() updates kernel_stack, so it seems that the old thread
gets this flag set, not new?

Even if this is correct, perhaps set_tsk_thread_flag(new) will look better?

The same for switch_fpu_finish(). I guess this should actually work after
this patch, because switch_fpu_finish() is called before
this_cpu_write(kernel_stack) too and thus both prepare/finish will use the
same thread_info, but this looks confusing at least.

> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -91,6 +91,7 @@ struct thread_info {
> #define TIF_SYSCALL_TRACEPOINT 28 /* syscall tracepoint instrumentation */
> #define TIF_ADDR32 29 /* 32-bit address space on 64 bits */
> #define TIF_X32 30 /* 32-bit native x86-64 binary */
> +#define TIF_LOAD_FPU 31 /* load FPU on return to userspace */

Well, the comment is wrong after this patch, but I see 4/11...

> #define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
> #define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
> @@ -115,6 +116,7 @@ struct thread_info {
> #define _TIF_SYSCALL_TRACEPOINT (1 << TIF_SYSCALL_TRACEPOINT)
> #define _TIF_ADDR32 (1 << TIF_ADDR32)
> #define _TIF_X32 (1 << TIF_X32)
> +#define _TIF_LOAD_FPU (1 << TIF_LOAD_FPU)
>
> /* work to do in syscall_trace_enter() */
> #define _TIF_WORK_SYSCALL_ENTRY \
> @@ -141,7 +143,7 @@ struct thread_info {
> /* Only used for 64 bit */
> #define _TIF_DO_NOTIFY_MASK \
> (_TIF_SIGPENDING | _TIF_MCE_NOTIFY | _TIF_NOTIFY_RESUME | \
> - _TIF_USER_RETURN_NOTIFY | _TIF_UPROBE)
> + _TIF_USER_RETURN_NOTIFY | _TIF_UPROBE | _TIF_LOAD_FPU)

This too. I mean, this change has no effect until 4/11.

Oleg.