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    SubjectRe: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching
    On Fri, Aug 09, 2019 at 08:38:14PM -0700, Paul E. McKenney wrote:
    > On Fri, Aug 09, 2019 at 10:42:32PM -0400, Joel Fernandes wrote:
    > > On Wed, Aug 07, 2019 at 10:52:15AM -0700, Paul E. McKenney wrote:
    > > [snip]
    > > > > > > @@ -3459,6 +3645,8 @@ void __init rcu_init(void)
    > > > > > > {
    > > > > > > int cpu;
    > > > > > >
    > > > > > > + kfree_rcu_batch_init();
    > > > > >
    > > > > > What happens if someone does a kfree_rcu() before this point? It looks
    > > > > > like it should work, but have you tested it?
    > > > > >
    > > > > > > rcu_early_boot_tests();
    > > > > >
    > > > > > For example, by testing it in rcu_early_boot_tests() and moving the
    > > > > > call to kfree_rcu_batch_init() here.
    > > > >
    > > > > I have not tried to do the kfree_rcu() this early. I will try it out.
    > > >
    > > > Yeah, well, call_rcu() this early came as a surprise to me back in the
    > > > day, so... ;-)
    > >
    > > I actually did get surprised as well!
    > >
    > > It appears the timers are not fully initialized so the really early
    > > kfree_rcu() call from rcu_init() does cause a splat about an initialized
    > > timer spinlock (even though future kfree_rcu()s and the system are working
    > > fine all the way into the torture tests).
    > >
    > > I think to resolve this, we can just not do batching until early_initcall,
    > > during which I have an initialization function which switches batching on.
    > > >From that point it is safe.
    >
    > Just go ahead and batch, but don't bother with the timer until
    > after single-threaded boot is done. For example, you could check
    > rcu_scheduler_active similar to how sync_rcu_exp_select_cpus() does.
    > (See kernel/rcu/tree_exp.h.)

    Cool, that works nicely and I tested it. Actually I made it such that we
    don't need to batch even, before the scheduler is up. I don't see any benefit
    of that unless we can see a kfree_rcu() flood happening that early at boot
    which seems highly doubtful as a real world case.

    > If needed, use an early_initcall() to handle the case where early boot
    > kfree_rcu() calls needed to set the timer but could not.

    And it would also need this complexity of early_initcall.

    > > Below is the diff on top of this patch, I think this should be good but let
    > > me know if anything looks odd to you. I tested it and it works.
    >
    > Keep in mind that a call_rcu() callback can't possibly be invoked until
    > quite some time after the scheduler is up and running. So it will be
    > a lot simpler to just skip setting the timer during early boot.

    Sure. Skipping batching would skip the timer too :-D

    If in the future, batching is needed this early, then I am happy to add an
    early_initcall to setup the timer for any batched calls that could not setup
    the timer. Hope that is ok with you?

    thanks,

    - Joel

    [snip]

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