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

[Andy Lutomirski]

On Tue, Jul 29, 2014 at 11:16 AM, Oleg Nesterov <oleg at redhat.com> wrote:
> On 07/29, Andy Lutomirski wrote:
>>
>> On Tue, Jul 29, 2014 at 10:31 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>> >
>> > 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.
>
>> 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