Track how the segcb list changes before/after acceleration, during queuing and during dequeuing. This has proved useful to discover an optimization to avoid unwanted GP requests when there are no callbacks accelerated. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- include/trace/events/rcu.h | 25 +++++++++++++++++++++++++ kernel/rcu/rcu_segcblist.c | 37 +++++++++++++++++++++++++++++++++++++ kernel/rcu/rcu_segcblist.h | 7 +++++++ kernel/rcu/tree.c | 24 ++++++++++++++++++++++++ 4 files changed, 93 insertions(+) diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h index 02dcd119f3263..a6d49864dcc27 100644 --- a/include/trace/events/rcu.h +++ b/include/trace/events/rcu.h @@ -507,6 +507,31 @@ TRACE_EVENT_RCU(rcu_callback, __entry->qlen) ); +TRACE_EVENT_RCU(rcu_segcb, + + TP_PROTO(const char *ctx, int *cb_count, unsigned long *gp_seq), + + TP_ARGS(ctx, cb_count, gp_seq), + + TP_STRUCT__entry( + __field(const char *, ctx) + __array(int, cb_count, 4) + __array(unsigned long, gp_seq, 4) + ), + + TP_fast_assign( + __entry->ctx = ctx; + memcpy(__entry->cb_count, cb_count, 4 * sizeof(int)); + memcpy(__entry->gp_seq, gp_seq, 4 * sizeof(unsigned long)); + ), + + TP_printk("%s cb_count: (DONE=%d, WAIT=%d, NEXT_READY=%d, NEXT=%d) " + "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx, + __entry->cb_count[0], __entry->cb_count[1], __entry->cb_count[2], __entry->cb_count[3], + __entry->gp_seq[0], __entry->gp_seq[1], __entry->gp_seq[2], __entry->gp_seq[3]) + +); + /* * Tracepoint for the registration of a single RCU callback of the special * kfree() form. The first argument is the RCU type, the second argument diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index 4782cf17bf4f9..036d4abac7c5a 100644 --- a/kernel/rcu/rcu_segcblist.c +++ b/kernel/rcu/rcu_segcblist.c @@ -316,6 +316,43 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp, WRITE_ONCE(rsclp->tails[i], &rsclp->head); } +/* + * Return how many CBs each segment along with their gp_seq values. + * + * This function is O(N) where N is the number of callbacks. Only used from + * tracing code which is usually disabled in production. + */ +#ifdef CONFIG_RCU_TRACE +void rcu_segcblist_countseq(struct rcu_segcblist *rsclp, + int cbcount[RCU_CBLIST_NSEGS], + unsigned long gpseq[RCU_CBLIST_NSEGS]) +{ + struct rcu_head **cur_tail, *h; + int i, c; + + for (i = 0; i < RCU_CBLIST_NSEGS; i++) + cbcount[i] = 0; + + cur_tail = &(rsclp->head); + + for (i = 0; i < RCU_CBLIST_NSEGS; i++) { + c = 0; + // List empty? + if (rsclp->tails[i] != cur_tail) { + // The loop skips the last node + c = 1; + for (h = *cur_tail; h->next != *(rsclp->tails[i]); h = h->next) { + c++; + } + } + + cbcount[i] = c; + gpseq[i] = rsclp->gp_seq[i]; + cur_tail = rsclp->tails[i]; + } +} +#endif + /* * Extract only those callbacks still pending (not yet ready to be * invoked) from the specified rcu_segcblist structure and place them in diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h index 5c293afc07b8e..0a8dbac438529 100644 --- a/kernel/rcu/rcu_segcblist.h +++ b/kernel/rcu/rcu_segcblist.h @@ -104,3 +104,10 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq); bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq); void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp, struct rcu_segcblist *src_rsclp); +#ifdef CONFIG_RCU_TRACE +void rcu_segcblist_countseq(struct rcu_segcblist *rsclp, + int cbcount[RCU_CBLIST_NSEGS], + unsigned long gpseq[RCU_CBLIST_NSEGS]); +#else +#define rcu_segcblist_countseq(...) +#endif diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index ebce14e470d0e..c61af6a33fbfd 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1407,6 +1407,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) { unsigned long gp_seq_req; bool ret = false; + int cbs[RCU_CBLIST_NSEGS]; + unsigned long gps[RCU_CBLIST_NSEGS]; rcu_lockdep_assert_cblist_protected(rdp); raw_lockdep_assert_held_rcu_node(rnp); @@ -1415,6 +1417,10 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) if (!rcu_segcblist_pend_cbs(&rdp->cblist)) return false; + /* Count CBs for tracing. */ + rcu_segcblist_countseq(&rdp->cblist, cbs, gps); + trace_rcu_segcb("SegCbPreAcc", cbs, gps); + /* * Callbacks are often registered with incomplete grace-period * information. Something about the fact that getting exact @@ -1434,6 +1440,11 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("AccWaitCB")); else trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("AccReadyCB")); + + /* Count CBs for tracing. */ + rcu_segcblist_countseq(&rdp->cblist, cbs, gps); + trace_rcu_segcb("SegCbPostAcc", cbs, gps); + return ret; } @@ -2316,6 +2327,8 @@ static void rcu_do_batch(struct rcu_data *rdp) struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl); long bl, count; long pending, tlimit = 0; + int cbs[RCU_CBLIST_NSEGS]; + unsigned long gps[RCU_CBLIST_NSEGS]; /* If no callbacks are ready, just return. */ if (!rcu_segcblist_ready_cbs(&rdp->cblist)) { @@ -2350,6 +2363,11 @@ static void rcu_do_batch(struct rcu_data *rdp) /* Invoke callbacks. */ tick_dep_set_task(current, TICK_DEP_BIT_RCU); rhp = rcu_cblist_dequeue(&rcl); + + /* Count CBs for tracing. */ + rcu_segcblist_countseq(&rdp->cblist, cbs, gps); + trace_rcu_segcb("SegCbDequeued", cbs, gps); + for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) { rcu_callback_t f; @@ -2808,6 +2826,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func) unsigned long flags; struct rcu_data *rdp; bool was_alldone; + int cbs[RCU_CBLIST_NSEGS]; + unsigned long gps[RCU_CBLIST_NSEGS]; /* Misaligned rcu_head! */ WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1)); @@ -2852,6 +2872,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func) trace_rcu_callback(rcu_state.name, head, rcu_segcblist_n_cbs(&rdp->cblist)); + /* Count CBs for tracing. */ + rcu_segcblist_countseq(&rdp->cblist, cbs, gps); + trace_rcu_segcb("SegCBQueued", cbs, gps); + /* Go handle any RCU core processing required. */ if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) && unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) { -- 2.27.0.111.gc72c7da667-goog