On Tue, Jul 29, 2014 at 10:31 AM, Oleg Nesterov [off-list ref] wrote:

On 07/29, Andy Lutomirski wrote:

quoted

On Tue, Jul 29, 2014 at 9:54 AM, Oleg Nesterov [off-list ref] wrote:

quoted

Yes, just to trigger the slow path, I guess.

quoted

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.

I don't get it.  context_tracking_is_enabled is global, and TIF_NOHZ
is per-task.  Isn't this stuff determined per-task or per-cpu or
something?

IOW, if one CPU is running something that's very heavily
userspace-oriented and another CPU is doing something syscall- or
sleep-heavy, then shouldn't only the first CPU end up paying the price
of context tracking?

quoted

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.

-- 
Andy Lutomirski
AMA Capital Management, LLC