On 07/29, Andy Lutomirski wrote: > > On Tue, Jul 29, 2014 at 11:16 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > On 07/29, Andy Lutomirski wrote: > >> > >> On Tue, Jul 29, 2014 at 10:31 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > >> > > >> > 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. I disagree. At least I disagree with that you should enforce this in syscall_trace_enter() paths, and in any case this has nothing to do with these changes. But again, I won't insist, so please forget. > So adding the > condition ought to be safe, barring dumb bugs in my code. Yes, I think it is technically correct. Oleg.
