Messages in this thread | | | Date | Tue, 15 May 2018 05:55:07 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works |
| |
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@joelfernandes.org> > > > > > --- > > > > > 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; > } > > > >
| |