On Sat 2013-05-04 17:23:01, Colin Cross wrote:
> On Sat, May 4, 2013 at 5:05 PM, Pavel Machek <pavel@xxxxxx> wrote:
> > Hi!
> >
> >> >> >> --- a/kernel/exit.c
> >> >> >> +++ b/kernel/exit.c
> >> >> >> @@ -835,7 +835,7 @@ void do_exit(long code)
> >> >> >> /*
> >> >> >> * Make sure we are holding no locks:
> >> >> >> */
> >> >> >> - debug_check_no_locks_held(tsk);
> >> >> >> + debug_check_no_locks_held();
> >> >> >
> >> >> > Is task guaranteed == current?
> >> >>
> >> >> Yes, the first line of do_exit is:
> >> >> struct task_struct *tsk = current;
> >> >
> >> > Aha, I understand it now.
> >> >
> >> > Accessing current is slower than local variable. So your "new" code
> >> > will work but will be slower. Please revert this part.
> >>
> >> Using current instead of passing in tsk was done at Andrew Morton's
> >> suggestion, and makes no difference from the freezer's perspective
> >> since it would have to use current to get the task to pass in, so I'm
> >> going to leave it as is.
> >
> > Well, current is:
> >
> > static inline struct thread_info *current_thread_info(void)
> > {
> > register unsigned long sp asm ("sp");
> > return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
> > }
> >
> > #define get_current() (current_thread_info()->task)
> >
> > #define current get_current()
> >
> > Instead of passing computed value to debug_check_no_locks_held(), you
> > force it to be computed again. do_exit() performance matters for
> > configure scripts, etc.
> >
> > I'd say it makes sense to keep the optimalization. akpm can correct
> > me.
>
> That translates to 3 instructions, with no memory accesses:
> c0008350: e1a0300d mov r3, sp
> c0008354: e3c32d7f bic r2, r3, #8128 ; 0x1fc0
> c0008358: e3c2203f bic r2, r2, #63 ; 0x3f
On ARM, you are right. It seems to have memory access on s390 and
x86-64. Ok, it is probably going to be in cache, but...
Pavel
--
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