On Tue, Jul 29, 2014 at 11:16 AM, Oleg Nesterov [off-list ref] wrote:

On 07/29, Andy Lutomirski wrote:

quoted

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

quoted

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.

Don't worry, you are not alone.

quoted

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?

Please see another email I sent to Frederic.

I'll add at least this argument in favor of my approach: if context
tracking works at all, then it had better not demand that syscall
entry call user_exit if TIF_NOHZ is *not* set.  So adding the
condition ought to be safe, barring dumb bugs in my code.

--Andy

Oleg.

-- 
Andy Lutomirski
AMA Capital Management, LLC