On Tue, May 15, 2018 at 12:08:01PM -0700, Paul E. McKenney wrote: > On Tue, May 15, 2018 at 11:41:15AM -0700, Joel Fernandes wrote: > > On Tue, May 15, 2018 at 05:55:07AM -0700, Paul E. McKenney wrote: > > > 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) <j...@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? ;-) > > :-D > > > > Ah I see what you're doing in that commit where you're moving the furthest > > request down to the leaves, so that would protect against the scenario I was > > describing and set the gp_seq_needed of the leaf. > > But I came up with a less baroque check for a grace period having started, > at which point the question becomes "Why not just do both?", especially > since a check for a grace period having started is satisfied by that > grace period's having completed, which means minimal added overhead. > Perhaps no added overhead for some compilers and architectures. > > Please see the end of this email for a prototype patch. > > > The code would be correct then, but one issue is it would shout out the > > 'Prestarted' tracepoint for 'c' when that's not really true.. > > > > rcu_seq_done(&rnp_root->gp_seq, c) > > > > translates to ULONG_CMP_GE(&rnp_root->gp_seq, c) > > > > which translates to the fact that c-1 completed. > > > > So in this case if rcu_seq_done returns true, then saying that c has been > > 'Prestarted' seems a bit off to me. It should be 'Startedleaf' or something > > since what we really are doing is just marking the leaf as you mentioned in > > the unlock_out part for a future start. > > Indeed, some of the tracing is not all that accurate. But the trace > message itself contains the information needed to work out why the > loop was exited, so perhaps something like 'EarlyExit'?
I think since you're now using rcu_seq_start to determine if c has started or completed since, the current 'Prestarted' trace will cover it. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 59a4f38edcffbef1521852fe3b26ed4ed85af16e > Author: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Date: Tue May 15 11:53:41 2018 -0700 > > rcu: Make rcu_start_this_gp() check for grace period already started > > In the old days of ->gpnum and ->completed, the code requesting a new > grace period checked to see if that grace period had already started, > bailing early if so. The new-age ->gp_seq approach instead checks > whether the grace period has already finished. A compensating change > pushed the requested grace period down to the bottom of the tree, thus > reducing lock contention and even eliminating it in some cases. But why > not further reduce contention, especially on large systems, by doing both, > especially given that the cost of doing both is extremely small? > > This commit therefore adds a new rcu_seq_started() function that checks > whether a specified grace period has already started. It then uses > this new function in place of rcu_seq_done() in the rcu_start_this_gp() > function's funnel locking code. > > Reported-by: Joel Fernandes <j...@joelfernandes.org> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 003671825d62..1c5cbd9d7c97 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -108,6 +108,15 @@ static inline unsigned long rcu_seq_current(unsigned > long *sp) > } > > /* > + * Given a snapshot from rcu_seq_snap(), determine whether or not the > + * corresponding update-side operation has started. > + */ > +static inline bool rcu_seq_started(unsigned long *sp, unsigned long s) > +{ > + return ULONG_CMP_LT((s - 1) & ~RCU_SEQ_STATE_MASK, READ_ONCE(*sp)); > +} > + > +/* > * Given a snapshot from rcu_seq_snap(), determine whether or not a > * full update-side operation has occurred. > */ > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 9e900c5926cc..ed69f49b7054 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1580,7 +1580,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, > struct rcu_data *rdp, > if (rnp_root != rnp) > raw_spin_lock_rcu_node(rnp_root); > if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) || > - rcu_seq_done(&rnp_root->gp_seq, c) || > + rcu_seq_started(&rnp_root->gp_seq, c) || Yes, this does exactly what I was wanting, thanks! I think this puts our discussion about this to rest :-) By the way I was starting to beautify this loop like below last week, with code comments. I felt it would be easier to parse this loop in the future for whoever was reading it. Are you interested in such a patch? If not, let me know and I'll drop this and focus on the other changes you requested. Something like... (just an example , actual code would be different) for (rnp_node = rnp; 1; rnp_node = rnp_node->parent) { int prestarted = 0; /* Acquire lock if non-leaf node */ if (rnp_node != rnp) raw_spin_lock_rcu_node(rnp_node); /* Has the GP asked been recorded as a future need */ if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start)) prestarted = 1; /* Has the GP requested for already been completed */ if (!prestarted && rcu_seq_completed(&rnp_node->gp_seq, gp_seq_start)) prestarted = 1; ... etc... if (prestarted) { trace_rcu_this_gp(rnp_node, rdp, gp_seq_start, TPS("Prestarted")); goto unlock_out; } thanks, - Joel