Re: WARNING: suspicious RCU usage - sdhci-pltfm: SDHCI platform and OF driver helper

[Paul E. McKenney]

On Wed, Sep 02, 2020 at 06:07:05PM +0200, Ulf Hansson wrote:
> On Wed, 2 Sep 2020 at 15:52, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Wed, Sep 02, 2020 at 08:49:11AM +0200, Ulf Hansson wrote:
> > > On Tue, 1 Sep 2020 at 17:00, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > [ . . . ]
> >
> > > > Here is the list, though it is early in the morning here:
> > > >
> > > > 1.      RCU_NONIDLE().
> > > >
> > > > 2.      Peter's patch, if it turns out to hoist your code out of what
> > > >         RCU considers to be the idle loop.
> > > >
> > > > 3.      If the problem is trace events, use the _rcuidle() variant of the
> > > >         trace event.  Instead of trace_blah(), use trace_blah_rcuidle().
> > > >
> > > > 4.      Switch from RCU (as in rcu_read_lock()) to SRCU (as in
> > > >         srcu_read_lock()).
> > > >
> > > > 5.      Take Peter's patch a step further, moving the rcu_idle_enter()
> > > >         and rcu_idle_exit() calls as needed.  But please keep in mind
> > > >         that these two functions require that irqs be disabled by their
> > > >         callers.
> > > >
> > > > 6.      If RCU_NONIDLE() in inconvenient due to early exits and such,
> > > >         you could use the rcu_irq_enter_irqson() and rcu_irq_exit_irqson()
> > > >         functions that it calls.
> > > >
> > > > Do any of those help?
> > >
> > > Yes, they will, in one way or the other. Thanks for providing me with
> > > all the available options.
> > >
> > > BTW, I still don't get what good rcu_idle_enter|exit() does, but I am
> > > assuming those need to be called at some point before the CPU goes to
> > > sleep.
> >
> > These functions allow RCU to leave idle CPUs undisturbed.  If they
> > were not invoked, RCU would periodically IPI idle CPUs to verify that
> > there were no RCU readers running on them.  This would be quite bad for
> > battery lifetime, among other things.  So the call to rcu_idle_enter()
> > tells RCU that it may safely completely ignore this CPU until its next
> > call to rcu_idle_exit().
> 
> Alright, thanks for explaining this, much appreciated.
> 
> So in one way, we would also like to call rcu_idle_enter(), as soon as
> we know there is no need for the RCU to be active. To prevent
> unnecessary IPIs I mean. :-)

Well, the IPIs don't happen until the better part of a second into
the grace period.  So delaying an rcu_idle_enter() a few microseconds,
as Peter Zijlstra is proposing, is absolutely no problem whatsoever.
And once the rcu_idle_enter() happens, the RCU grace-period kthread's next
scan of the CPUs will see that this CPU needs to be ignored, so no more
IPIs for it until it does the next rcu_idle_exit(), rcu_irq_enter(),
or any of a number of other things that cause RCU to once again pay
attention to that CPU.

							Thanx, Paul