On Mon, 22 May 2017 17:00:36 -0700 "Paul E. McKenney" <[email protected]> wrote:
> On Fri, May 19, 2017 at 12:06:09PM -0700, Paul E. McKenney wrote: > > On Fri, May 19, 2017 at 10:23:31AM -0400, Steven Rostedt wrote: > > > On Fri, 19 May 2017 10:04:21 -0400 > > > Steven Rostedt <[email protected]> wrote: > > > > > > > On Fri, 19 May 2017 06:35:50 -0700 > > > > "Paul E. McKenney" <[email protected]> wrote: > > > > > > > > > Simpler would be better! > > > > > > > > > > However, is it really guaranteed that one SCHED_IDLE thread > > > > > cannot preempt another? If not, then the trampoline-freeing > > > > > SCHED_IDLE thread might preempt some other SCHED_IDLE thread > > > > > in the middle of a trampoline. I am not seeing anything that > > > > > prevents such preemption, but it is rather early local time, > > > > > so I could easily be missing something. > > > > > > > > > > However, if SCHED_IDLE threads cannot preempt other threads, > > > > > even other SCHED_IDLE threads, then your approach sounds > > > > > quite promising to me. > > > > > > > > > > Steve, Peter, thoughts? > > > > > > > > SCHED_IDLE is the swapper task. There's one on each CPU, and > > > > they don't migrate. And they only get called when there's no > > > > other task running. > > > > > > Peter just "schooled" me on IRC. I stand corrected (and he may > > > respond to this email too). I guess any task can become > > > SCHED_IDLE. > > > > > > But that just makes this an even less likely option for > > > synchronize_rcu_tasks(). > > > > Hmmm... The goal is to make sure that any task that was preempted > > or running at a given point in time passes through a voluntary > > context switch (or userspace execution, or, ...). > > > > What is the simplest way to get this job done? To Ingo's point, I > > bet that there is a simpler way than the current TASKS_RCU > > implementation. > > > > Ingo, if I make it fit into 100 lines of code, would you be OK with > > it? I probably need a one-line hook at task-creation time and > > another at task-exit time, if that makes a difference. > > And please see below for such a patch, which does add (just barely) > fewer than 100 lines net. > > Unfortunately, it does not work, as I should have known ahead of time > from the dyntick-idle experience. Not all context switches go through > context_switch(). :-/ > > I believe this is fixable, more or less like dyntick-idle's > half-interrupts were fixable, but it will likely be a few days. Not > clear whether the result will be simpler than current TASKS_RCU, but > there is only one way to find out. ;-) > Hi Paul, I'm currently traveling and don't have the energy to look at code at the moment. Hopefully, I can look at this more on Thursday or Friday. Thanks! -- Steve
