On 07/29, Andy Lutomirski wrote:
>
> On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> >
> > Yes, just to trigger the slow path, I guess.
> >
> >> I'll update the code to call user_exit iff TIF_NOHZ is
> >> set.
> >
> > Or perhaps it would be better to not add another user of this (strange) flag
> > and just call user_exit() unconditionally(). But, yes, you need to use
> > from "work = flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ)" then.\
>
> user_exit looks slow enough to me that a branch to try to avoid it may
> be worthwhile. I bet that explicitly checking the flag is
> actually both faster and clearer.
I don't think so (unless I am confused again), note that user_exit() uses
jump label. But this doesn't matter. I meant that we should avoid TIF_NOHZ
if possible because I think it should die somehow (currently I do not know
how ;). And because it is ugly to check the same condition twice:
if (work & TIF_NOHZ) {
// user_exit()
if (context_tracking_is_enabled())
context_tracking_user_exit();
}
TIF_NOHZ is set if and only if context_tracking_is_enabled() is true.
So I think that
work = current_thread_info()->flags & (_TIF_WORK_SYSCALL_ENTRY & ~TIF_NOHZ);
user_exit();
looks a bit better. But I won't argue.
> That's what I did for v4.
I am going to read it today. Not that I think I can help or find something
wrong.
Oleg.
