Re: [PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Paul,

On Thu, 13 May 2021 12:15:39 -0700
"Paul E. McKenney" <paulmck@kernel.org> 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@kernel.org> 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@kernel.org> 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@kernel.org>

Thank you,


-- 
Masami Hiramatsu <mhiramat@kernel.org>