Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works

[Paul E. McKenney]

On Tue, May 15, 2018 at 12:02:43AM -0700, Joel Fernandes wrote:
> Hi Paul,
> Good morning, hope you're having a great Tuesday. I managed to find some
> evening hours today to dig into this a bit more.
> 
> On Mon, May 14, 2018 at 08:59:52PM -0700, Paul E. McKenney wrote:
> > On Mon, May 14, 2018 at 06:51:33PM -0700, Joel Fernandes wrote:
> > > On Mon, May 14, 2018 at 10:38:16AM -0700, Paul E. McKenney wrote:
> > > > On Sun, May 13, 2018 at 08:15:34PM -0700, Joel Fernandes (Google) wrote:
> > > > > rcu_seq_snap may be tricky for someone looking at it for the first time.
> > > > > Lets document how it works with an example to make it easier.
> > > > > 
> > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > > > > ---
> > > > >  kernel/rcu/rcu.h | 24 +++++++++++++++++++++++-
> > > > >  1 file changed, 23 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > > > > index 003671825d62..fc3170914ac7 100644
> > > > > --- a/kernel/rcu/rcu.h
> > > > > +++ b/kernel/rcu/rcu.h
> > > > > @@ -91,7 +91,29 @@ static inline void rcu_seq_end(unsigned long *sp)
> > > > >  	WRITE_ONCE(*sp, rcu_seq_endval(sp));
> > > > >  }
> > > > > 
> > > > > -/* Take a snapshot of the update side's sequence number. */
> > > > > +/*
> > > > > + * Take a snapshot of the update side's sequence number.
> > > > > + *
> > > > > + * This function predicts what the grace period number will be the next
> > > > > + * time an RCU callback will be executed, given the current grace period's
> > > > > + * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is
> > > > > + * already in progress.
> > > > 
> > > > How about something like this?
> > > > 
> > > > 	This function returns the earliest value of the grace-period
> > > > 	sequence number that will indicate that a full grace period has
> > > > 	elapsed since the current time.  Once the grace-period sequence
> > > > 	number has reached this value, it will be safe to invoke all
> > > > 	callbacks that have been registered prior to the current time.
> > > > 	This value is the current grace-period number plus two to the
> > > > 	power of the number of low-order bits reserved for state, then
> > > > 	rounded up to the next value in which the state bits are all zero.
> > > 
> > > This makes sense too, but do you disagree with what I said?
> > 
> > In a pedantic sense, definitely.  RCU callbacks are being executed pretty
> > much all the time on a busy system, so it is only the recently queued
> > ones that are guaranteed to be deferred that long.  And my experience
> > indicates that someone really will get confused by that distinction,
> > so I feel justified in being pedantic in this case.
> 
> Ok I agree, I'll include your comment above.
> 
> > > Also just to let you know, thanks so much for elaborately providing an
> > > example on the other thread where we are discussing the rcu_seq_done check. I
> > > will take some time to trace this down and see if I can zero in on the same
> > > understanding as yours.
> > > 
> > > I get why we use rcu_seq_snap there in rcu_start_this_gp but the way it its
> > > used is 'c' is the requested GP obtained from _snap, and we are comparing that with the existing
> > > rnp->gp_seq in rcu_seq_done.  When that rnp->gp_seq reaches 'c', it only
> > > means rnp->gp_seq is done, it doesn't tell us if 'c' is done which is what
> > > we were trying to check in that loop... that's why I felt that check wasn't
> > > correct - that's my (most likely wrong) take on the matter, and I'll get back
> > > once I trace this a bit more hopefully today :-P
> > 
> > If your point is that interrupts are disabled throughout, so there isn't
> > much chance of the grace period completing during that time, you are
> > mostly right.  The places you might not be right are the idle loop and
> > offline CPUs.  And yes, call_rcu() doesn't like queuing callbacks onto
> > offline CPUs, but IIRC it is just fine in the case where callbacks have
> > been offloaded from that CPU.
> > 
> > And if you instead say that "c" is the requested final ->gp_seq value
> > obtained from _snap(), the thought process might go more easily.
> 
> Yes I agree with c being the requested final value which is the GP for which
> the callbacks will be queued. At the end of the GP c, the callbacks will have
> executed.
> 
> About the rcu_seq_done check and why I believe its not right to use it in
> that funnel locking loop, if you could allow me to try argument my point from
> a different angle...
> 
> We agreed that the way gp_seq numbers work and are compared with each other
> to identify if a GP is elapsed or not, is different from the way the previous
> numbers (gp_num) were compared.
> 
> Most notably, before the gp_seq conversions - inorder to start a GP, we were
> doing gp_num += 1, and completed had to catch up to gp_num + 1 to mark the
> end.
> 
> Now with gp_seq, for a gp to start, we don't do the "+1", we just set the
> state bits. To mark the end, we clear the state bits and increment the gp_num
> part of gp_seq.
> 
> However, in the below commit 12d6c129fd0a ("rcu: Convert grace-period
> requests to ->gp_seq"). You did a one-to-one replacement of the ULONG_CMP_GE
> with rcu_seq_done. You did so even though the gp_seq numbers work differently
> from previously used numbers (gp_num and completed).
> 
> I would then argue that because of the differences above, a one-to-one
> replacement of the ULONG_CMP_GE with the rcu_seq_done wouldn't make sense.
> 
> I argue this because, in previous code - the ULONG_CMP_GE made sense for the gp_num
> way of things because, if c == gp_num, that means that :
>  - c started already
>  - c has finished.
>  Which worked correctly, because we have nothing to do and we can bail
>  without setting any flag.
> 
>  Where as now, with the gp_seq regime, c == gp_seq means:
>  - c-1 finished   (I meant -1 subtracted from the gp_num part of c)
>  This would cause us to bail without setting any flag for starting c.
> 
>  I did some tracing and I could never hit the rcu_seq_done check because it
>  never happens in my tracing that _snap returned something for which
>  rcu_seq_done returned true, so I'm not sure if this check is needed, but
>  you're the expert ;)
> 
> @@ -1629,16 +1583,16 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>          * not be released.
>          */
>         raw_lockdep_assert_held_rcu_node(rnp);
> +       WARN_ON_ONCE(c & 0x2); /* Catch any lingering use of ->gpnum. */
> +       WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq)); /* Catch any ->completed/->gp_seq mismatches. */
>         trace_rcu_this_gp(rnp, rdp, c, TPS("Startleaf"));
>         for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
>                 if (rnp_root != rnp)
>                         raw_spin_lock_rcu_node(rnp_root);
> -               WARN_ON_ONCE(ULONG_CMP_LT(rnp_root->gpnum +
> -                                         need_future_gp_mask(), c));
>                 if (need_future_gp_element(rnp_root, c) ||
> -                   ULONG_CMP_GE(rnp_root->gpnum, c) ||
> +                   rcu_seq_done(&rnp_root->gp_seq, c) ||
> 
>                      ^^^^
> 		     A direct replacement of ULONG_CMP_GE is bit weird?  It
> 		     means we bail out if c-1 completed, and we don't set any
> 		     flag for starting c. That could result in the clean up
> 		     never starting c?

Ah, I see what you are getting at now.

What I do instead in 334dac2da529 ("rcu: Make rcu_nocb_wait_gp() check
if GP already requested") is to push the request down to the leaves of
the tree and to the rcu_data structure.  Once that commit is in place,
the check for the grace period already being in progress isn't all
that helpful, though I suppose that it could be added.  One way to
do that would be to replace "rcu_seq_done(&rnp_root->gp_seq, c)" with
ULONG_CMP_GE(rnp_root->gpnum, (c - RCU_SEQ_STATE_MASK))", but that seems
a bit baroque to me.

The point of the rcu_seq_done() is to catch long delays, but given the
current implementation, the fact that interrupts are disabled across
all calls should prevent the rcu_seq_done() from ever returning true.
(Famous last words!)  So, yes, it could be removed, in theory, at least.
At least until the real-time guys force me to come up with a way to
run this code with interrupts enabled (hopefully never!).

If I were to do that, I would first wrap it with a WARN_ON_ONCE() and
leave it that way for an extended period of testing.  Yes, I am paranoid.
Why do you ask?  ;-)

							Thanx, Paul

>                     (rnp != rnp_root &&
> -                    rnp_root->gpnum != rnp_root->completed)) {
> +                    rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
>                         trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
>                         goto unlock_out;
>                 }
> 
> 
> 
>