On Fri, May 14, 2021 at 03:04:31PM +0900, Masami Hiramatsu wrote: > Hi Paul, > > On Thu, 13 May 2021 12:15:39 -0700 > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > On Fri, May 14, 2021 at 02:49:12AM +0900, Masami Hiramatsu wrote: > > > On Thu, 13 May 2021 07:21:10 -0700 > > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > > > > > On Thu, May 13, 2021 at 03:54:17PM +0900, Masami Hiramatsu wrote: > > > > > Hi Paul, > > > > > > > > > > On Wed, 12 May 2021 11:27:46 -0700 > > > > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > > > > > > > > > Heavy networking load can cause a CPU to execute continuously and > > > > > > indefinitely within ksoftirqd, in which case there will be no voluntary > > > > > > task switches and thus no RCU-tasks quiescent states. This commit > > > > > > therefore causes the exiting rcu_softirq_qs() to provide an RCU-tasks > > > > > > quiescent state. > > > > > > > > > > > > This of course means that __do_softirq() and its callers cannot be > > > > > > invoked from within a tracing trampoline. > > > > > > > > > > I would like to confirm that you mean "tracing trampoline" here is > > > > > the code on the trampoline buffer, not the handler code which is > > > > > invoked from the trampoline buffer but it is protected by preempt_disable(), > > > > > am I understand correctly? > > > > > > > > Maybe? ;-) > > > > > > > > If the handler code is invoked from the trampoline buffer, but > > > > returns somewhere else, then it is OK for the handler code to invoke > > > > __do_softirq() or its callers. > > > > > > > > In addition, if the handler code is invoked from the trampoline buffer is > > > > guaranteed never to be running in the context of the ksoftirqd kthread, > > > > then it is also OK for the handler code to invoke __do_softirq() or > > > > its callers. > > > > > > > > Otherwise, if the handler code might return back into the trampoline > > > > buffer and if that code might be running in the context of the ksoftirqd > > > > kthread, invoking __do_softirq() or one of its callers could result in > > > > the trampoline buffer no longer being there when it was returned to. > > > > > > Hmm, the optprobe may be involved in this case. It always return to > > > the trampoline and handler does not disable irqs (only disable preempt). > > > BTW, what will call the __do_softirq()? Is hardirq safe? > > > > As long as your code does not explicitly call __do_softirq() or one of > > its callers, you should be OK. > > > > Let's suppose that your code takes a hardirq from ksoftirqd context. > > In that case, the return-from-irq path will notice the ksoftirqd > > context and refrain from calling __do_softirqd(). Life is good. > > (See the invoke_softirq() function for more detail.) > > > > On the other hand, if your code takes a hardirq from some non-ksoftirqd > > context, and if this hardirq decides to handle softirqs on exit > > from the hardirq, the "__this_cpu_read(ksoftirqd) == current" within > > __do_softirq() will fail, so that rcu_softirq_qs() will not be called. > > Life is still good. > > Ah, OK. This is good. > > > > > Either way, as long as your handler does not explicitly invoke > > __do_softirq(), life is good. > > There should be no such code, I hope. > > > > > The bad case is when you instrument a function that is invoked in the > > context of a ksoftirqd kthread, and the corresponding handler (or > > some function that the handler explicitly calls) directly invokes > > __do_softirq() or one of its caller. > > > > Is that more helpful? > > OK, I got it. So it would be better to be commented later. > But anyway I can't imagine that there is any reason to call > __do_softirq() inside kprobe handler :) > > Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> I will apply on the next rebase, thank you! Thanx, Paul
