Messages in this thread | | | Date | Tue, 13 Jan 2015 16:53:35 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to userspace |
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On 01/11, riel@redhat.com wrote: > > @@ -382,6 +382,7 @@ static inline void drop_init_fpu(struct task_struct *tsk) > else > fxrstor_checking(&init_xstate_buf->i387); > } > + clear_thread_flag(TIF_LOAD_FPU);
OK, in this case tsk should be current. Still I think clear_tsk_thread_flag() will look more consistent.
> @@ -435,24 +436,32 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc > prefetch(new->thread.fpu.state);
I am wondering if these prefetch()es in switch_fpu_prepare() make sense after this patch.
> + } else > + /* > + * The new task does not want an FPU state restore, > + * and may not even have an FPU state. However, the > + * old task may have left TIF_LOAD_FPU set. > + * Clear it to avoid trouble. > + * > + * CR0.TS is still set from a previous FPU switch > + */ > + clear_thread_flag(TIF_LOAD_FPU);
I got lost ;) Simply can't understand what this change tries to do.
And it looks "obviously wrong"... OK, suppose that a TIF_LOAD_FPU task schedules before it returns to user mode (and calls switch_fpu_finish). Why should we clear its flag if the new task doesn't want FPU ?
"CR0.TS is still set" is not true if use_eager_fpu()... OTOH, .TS can be also set even if preload == T above, when we set TIF_LOAD_FPU.
> -static inline void switch_fpu_finish(struct task_struct *new) > +static inline void switch_fpu_finish(void) > { > - if (test_and_clear_thread_flag(TIF_LOAD_FPU)) { > - __thread_fpu_begin(new); > - if (unlikely(restore_fpu_checking(new))) > - drop_init_fpu(new); > - } > + struct task_struct *tsk = current; > + > + __thread_fpu_begin(tsk); > + > + if (unlikely(restore_fpu_checking(tsk))) > + drop_init_fpu(tsk); > }
Again, I am totally confused. After this patch the usage of set_thread_flag() in switch_fpu_prepare() becomes wrong (see my reply to 2/11), but it is quite possible I misunderstood these patches.
Oleg.
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