Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Paul E. McKenney
2023-10-21T02:08:09Z

On Fri, Oct 20, 2023 at 06:05:21PM -0700, Ankur Arora wrote:
>
> Paul E. McKenney <paulmck@kernel.org> writes:
>
> > On Fri, Oct 20, 2023 at 03:56:38PM -0700, Ankur Arora wrote:
> >>
> >> Paul E. McKenney <paulmck@kernel.org> writes:
> >>
> >> > Thomas!
> >> >
> >> > On Thu, Oct 19, 2023 at 02:21:35AM +0200, Thomas Gleixner wrote:
> >> >> Paul!
> >> >>
> >> >> On Wed, Oct 18 2023 at 10:19, Paul E. McKenney wrote:
> >> >> > On Wed, Oct 18, 2023 at 03:16:12PM +0200, Thomas Gleixner wrote:
> >> >> >> On Tue, Oct 17 2023 at 18:03, Paul E. McKenney wrote:
> >> >> >> In the end there is no CONFIG_PREEMPT_XXX anymore. The only knob
> >> >> >> remaining would be CONFIG_PREEMPT_RT, which should be renamed to
> >> >> >> CONFIG_RT or such as it does not really change the preemption
> >> >> >> model itself. RT just reduces the preemption disabled sections with the
> >> >> >> lock conversions, forced interrupt threading and some more.
> >> >> >
> >> >> > Again, please, no.
> >> >> >
> >> >> > There are situations where we still need rcu_read_lock() and
> >> >> > rcu_read_unlock() to be preempt_disable() and preempt_enable(),
> >> >> > repectively. Those can be cases selected only by Kconfig option, not
> >> >> > available in kernels compiled with CONFIG_PREEMPT_DYNAMIC=y.
> >> >>
> >> >> Why are you so fixated on making everything hardcoded instead of making
> >> >> it a proper policy decision problem. See above.
> >> >
> >> > Because I am one of the people who will bear the consequences.
> >> >
> >> > In that same vein, why are you so opposed to continuing to provide
> >> > the ability to build a kernel with CONFIG_PREEMPT_RCU=n? This code
> >> > is already in place, is extremely well tested, and you need to handle
> >> > preempt_disable()/preeempt_enable() regions of code in any case. What is
> >> > the real problem here?
> >>
>
> [ snip ]
>
> >> As far as I can tell (which isn't all that far), TREE_RCU=y makes strictly
> >> stronger forward progress guarantees with respect to rcu readers (in
> >> that they can't be preempted.)
> >
> > TREE_RCU=y is absolutely required if you want a kernel to run on a system
> > with more than one CPU, and for that matter, if you want preemptible RCU,
> > even on a single-CPU system.
> >
> >> So, can PREEMPTION=y run with, say TREE_RCU=y? Or maybe I'm missing something
> >> obvious there.
> >
> > If you meant to ask about PREEMPTION and PREEMPT_RCU, in theory, you
> > can run any combination:
>
> Sorry, yes I did. Should have said "can PREEMPTION=y run with, (TREE_RCU=y,
> PREEMPT_RCU=n).
>
> > PREEMPTION && PREEMPT_RCU: This is what we use today for preemptible
> > kernels, so this works just fine (famous last words).
> >
> > PREEMPTION && !PREEMPT_RCU: A preemptible kernel with non-preemptible
> > RCU, so that rcu_read_lock() is preempt_disable() and
> > rcu_read_unlock() is preempt_enable(). This should just work,
> > except for the fact that cond_resched() disappears, which
> > stymies some of RCU's forward-progress mechanisms. And this
> > was the topic of our earlier discussion on this thread. The
> > fixes should not be too hard.
> >
> > Of course, this has not been either tested or used for at least
> > eight years, so there might be some bitrot. If so, I will of
> > course be happy to help fix it.
> >
> >
> > !PREEMPTION && PREEMPT_RCU: A non-preemptible kernel with preemptible
> > RCU. Although this particular combination of Kconfig
> > options has not been tested for at least eight years, giving
> > a kernel built with CONFIG_PREEMPT_DYNAMIC=y the preempt=none
> > kernel boot parameter gets you pretty close. Again, there is
> > likely to be some bitrot somewhere, but way fewer bits to rot
> > than for PREEMPTION && !PREEMPT_RCU. Outside of the current
> > CONFIG_PREEMPT_DYNAMIC=y case, I don't see the need for this
> > combination, but if there is a need and if it is broken, I will
> > be happy to help fix it.
> >
> > !PREEMPTION && !PREEMPT_RCU: A non-preemptible kernel with non-preemptible
> > RCU, which is what we use today for non-preemptible kernels built
> > with CONFIG_PREEMPT_DYNAMIC=n. So to repeat those famous last
> > works, this works just fine.
> >
> > Does that help, or am I missing the point of your question?
>
> It does indeed. What I was going for, is that this series (or, at
> least my adaptation of TGLX's PoC) wants to keep CONFIG_PREEMPTION
> in spirit, while doing away with it as a compile-time config option.
>
> That it does, as TGLX mentioned upthread, by moving all of the policy
> to the scheduler, which can be tuned by user-space (via sched-features.)
>
> So, my question was in response to this:
>
> >> > In that same vein, why are you so opposed to continuing to provide
> >> > the ability to build a kernel with CONFIG_PREEMPT_RCU=n? This code
> >> > is already in place, is extremely well tested, and you need to handle
> >> > preempt_disable()/preeempt_enable() regions of code in any case. What is
> >> > the real problem here?
>
> Based on your response the (PREEMPT_RCU=n, TREE_RCU=y) configuration
> seems to be eminently usable with this configuration.
>
> (Or maybe I'm missed the point of that discussion.)
>
> On a related note, I had started rcutorture on a (PREEMPTION=y, PREEMPT_RCU=n,
> TREE_RCU=y) kernel some hours ago. Nothing broken (yet!).

Thank you, and here is hoping! ;-)

Thanx, Paul