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SubjectRe: Consolidating RCU-bh, RCU-preempt, and RCU-sched
On Fri, Jul 13, 2018 at 11:47:18AM +0800, Lai Jiangshan wrote:
> On Fri, Jul 13, 2018 at 8:02 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > Hello!
> >
> > I now have a semi-reasonable prototype of changes consolidating the
> > RCU-bh, RCU-preempt, and RCU-sched update-side APIs in my -rcu tree.
> > There are likely still bugs to be fixed and probably other issues as well,
> > but a prototype does exist.
> >
> > Assuming continued good rcutorture results and no objections, I am
> > thinking in terms of this timeline:
> >
> > o Preparatory work and cleanups are slated for the v4.19 merge window.
> >
> > o The actual consolidation and post-consolidation cleanup is slated
> > for the merge window after v4.19 (v5.0?). These cleanups include
> > the replacements called out below within the RCU implementation
> > itself (but excluding kernel/rcu/sync.c, see question below).
> >
> > o Replacement of now-obsolete update APIs is slated for the second
> > merge window after v4.19 (v5.1?). The replacements are currently
> > expected to be as follows:
> >
> > synchronize_rcu_bh() -> synchronize_rcu()
> > synchronize_rcu_bh_expedited() -> synchronize_rcu_expedited()
> > call_rcu_bh() -> call_rcu()
> > rcu_barrier_bh() -> rcu_barrier()
> > synchronize_sched() -> synchronize_rcu()
> > synchronize_sched_expedited() -> synchronize_rcu_expedited()
> > call_rcu_sched() -> call_rcu()
> > rcu_barrier_sched() -> rcu_barrier()
> > get_state_synchronize_sched() -> get_state_synchronize_rcu()
> > cond_synchronize_sched() -> cond_synchronize_rcu()
> > synchronize_rcu_mult() -> synchronize_rcu()
> >
> > I have done light testing of these replacements with good results.
> >
> > Any objections to this timeline?
> >
> > I also have some questions on the ultimate end point. I have default
> > choices, which I will likely take if there is no discussion.
> >
> > o
> > Currently, I am thinking in terms of keeping the per-flavor
> > read-side functions. For example, rcu_read_lock_bh() would
> > continue to disable softirq, and would also continue to tell
> > lockdep about the RCU-bh read-side critical section. However,
> > synchronize_rcu() will wait for all flavors of read-side critical
> > sections, including those introduced by (say) preempt_disable(),
> > so there will no longer be any possibility of mismatching (say)
> > RCU-bh readers with RCU-sched updaters.
> >
> > I could imagine other ways of handling this, including:
> >
> > a. Eliminate rcu_read_lock_bh() in favor of
> > local_bh_disable() and so on. Rely on lockdep
> > instrumentation of these other functions to identify RCU
> > readers, introducing such instrumentation as needed. I am
> > not a fan of this approach because of the large number of
> > places in the Linux kernel where interrupts, preemption,
> > and softirqs are enabled or disabled "behind the scenes".
> >
> > b. Eliminate rcu_read_lock_bh() in favor of rcu_read_lock(),
> > and required callers to also disable softirqs, preemption,
> > or whatever as needed. I am not a fan of this approach
> > because it seems a lot less convenient to users of RCU-bh
> > and RCU-sched.
> >
> > At the moment, I therefore favor keeping the RCU-bh and RCU-sched
> > read-side APIs. But are there better approaches?
>
> Hello, Paul
>
> Since local_bh_disable() will be guaranteed to be protected by RCU
> and more general. I'm afraid it will be preferred over
> rcu_read_lock_bh() which will be gradually being phased out.
>
> In other words, keeping the RCU-bh read-side APIs will be a slower
> version of the option A. So will the same approach for the RCU-sched.
> But it'll still be better than the hurrying option A, IMHO.

I am OK with the read-side RCU-bh and RCU-sched interfaces going away,
it is just that I am not willing to put all that much effort into
it myself. ;-)

Unless there is a good reason for me to hurry it along, of course.

Thanx, Paul

> Thanks,
> Lai
>
> >
> > o How should kernel/rcu/sync.c be handled? Here are some
> > possibilities:
> >
> > a. Leave the full gp_ops[] array and simply translate
> > the obsolete update-side functions to their RCU
> > equivalents.
> >
> > b. Leave the current gp_ops[] array, but only have
> > the RCU_SYNC entry. The __INIT_HELD field would
> > be set to a function that was OK with being in an
> > RCU read-side critical section, an interrupt-disabled
> > section, etc.
> >
> > This allows for possible addition of SRCU functionality.
> > It is also a trivial change. Note that the sole user
> > of sync.c uses RCU_SCHED_SYNC, and this would need to
> > be changed to RCU_SYNC.
> >
> > But is it likely that we will ever add SRCU?
> >
> > c. Eliminate that gp_ops[] array, hard-coding the function
> > pointers into their call sites.
> >
> > I don't really have a preference. Left to myself, I will be lazy
> > and take option #a. Are there better approaches?
> >
> > o Currently, if a lock related to the scheduler's rq or pi locks is
> > held across rcu_read_unlock(), that lock must be held across the
> > entire read-side critical section in order to avoid deadlock.
> > Now that the end of the RCU read-side critical section is
> > deferred until sometime after interrupts are re-enabled, this
> > requirement could be lifted. However, because the end of the RCU
> > read-side critical section is detected sometime after interrupts
> > are re-enabled, this means that a low-priority RCU reader might
> > remain priority-boosted longer than need be, which could be a
> > problem when running real-time workloads.
> >
> > My current thought is therefore to leave this constraint in
> > place. Thoughts?
> >
> > Anything else that I should be worried about? ;-)
> >
> > Thanx, Paul
> >
>

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