Re: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases

[Andy Lutomirski]

On Tue, Jul 29, 2014 at 10:31 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> 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.

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?

>
>> 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
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