Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

Joel Fernandes
2022-09-27T13:05:48Z

On Mon, Sep 26, 2022 at 08:22:46PM -0700, Paul E. McKenney wrote:
[..]
> > > > >>> --- a/kernel/workqueue.c
> > > > >>> +++ b/kernel/workqueue.c
> > > > >>> @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)
> > > > >>>
> > > > >>> if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
> > > > >>> rwork->wq = wq;
> > > > >>> - call_rcu(&rwork->rcu, rcu_work_rcufn);
> > > > >>> + call_rcu_flush(&rwork->rcu, rcu_work_rcufn);
> > > > >>> return true;
> > > > >>> }
> > > > >>>
> > > > >>> <snip>
> > > > >>>
> > > > >>> ?
> > > > >>>
> > > > >>> But it does not fully solve my boot-up issue. Will debug tomorrow further.
> > > > >>
> > > > >> Ah, but at least its progress, thanks. Could you send me a patch to include
> > > > >> in the next revision with details of this?
> > > > >>
> > > > >>>> Might one more proactive approach be to use Coccinelle to locate such
> > > > >>>> callback functions? We might not want -all- callbacks that do wakeups
> > > > >>>> to use call_rcu_flush(), but knowing which are which should speed up
> > > > >>>> slow-boot debugging by quite a bit.
> > > > >>>>
> > > > >>>> Or is there a better way to do this?
> > > > >>>>
> > > > >>> I am not sure what Coccinelle is. If we had something automated that measures
> > > > >>> a boot time and if needed does some profiling it would be good. Otherwise it
> > > > >>> is a manual debugging mainly, IMHO.
> > > > >>
> > > > >> Paul, What about using a default-off kernel CONFIG that splats on all lazy
> > > > >> call_rcu() callbacks that do a wake up. We could use the trace hooks to do it
> > > > >> in kernel I think. I can talk to Steve to get ideas on how to do that but I
> > > > >> think it can be done purely from trace events (we might need a new
> > > > >> trace_end_invoke_callback to fire after the callback is invoked). Thoughts?
> > > > >
> > > > > Could you look for wakeups invoked between trace_rcu_batch_start() and
> > > > > trace_rcu_batch_end() that are not from interrupt context? This would
> > > > > of course need to be associated with a task rather than a CPU.
> > > >
> > > > Yes this sounds good, but we also need to know if the callbacks are
> > > > lazy or not since wake-up is ok from a non lazy one. I think I’ll
> > > > need a table to track that at queuing time.
> > >
> > > Agreed.
> > >
> > > > > Note that you would need to check for wakeups from interrupt handlers
> > > > > even with the extra trace_end_invoke_callback(). The window where an
> > > > > interrupt handler could do a wakeup would be reduced, but not eliminated.
> > > >
> > > > True! Since this is a debugging option, can we not just disable interrupts across callback invocation?
> > >
> > > Not without terminally annoying lockdep, at least for any RCU callbacks
> > > doing things like spin_lock_bh().
> > >
> >
> > Sorry if my last email bounced. Looks like my iPhone betrayed me this once ;)
> >
> > I was thinking something like this:
> > 1. Put a flag in rcu_head to mark CBs as lazy.
> > 2. Add a trace_rcu_invoke_callback_end() trace point.
> >
> > Both #1 and #2 can be a debug CONFIG option. #2 can be a tracepoint and not
> > exposed if needed.
> >
> > 3. Put an in-kernel probe on both trace_rcu_invoke_callback_start() and
> > trace_rcu_invoke_callback_end(). In the start probe, set a per-task flag if
> > the current CB is lazy. In the end probe, clear it.
> >
> > 4. Put an in-kernel probe on trace_rcu_sched_wakeup().
> >
> > Splat in the wake up probe if:
> > 1. Hard IRQs are on.
> > 2. The per-cpu flag is set.
> >
> > #3 actually does not even need probes if we can directly call the functions
> > from the rcu_do_batch() function.
>
> This is fine for an experiment or a debugging session, but a solution
> based totally on instrumentation would be better for production use.

Maybe we can borrow the least-significant bit of rhp->func to mark laziness?
Then it can be production as long as we're ok with the trace_sched_wakeup
probe.

thanks,

- Joel