Messages in this thread | | | Subject | Re: [PATCH v6 3/4] rcu/trace: Add tracing for how segcb list changes | From | Neeraj Upadhyay <> | Date | Wed, 14 Oct 2020 20:52:17 +0530 |
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On 9/23/2020 8:52 PM, Joel Fernandes (Google) wrote: > 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. The overhead is minimal as > each segment's length is now stored in the respective segment. > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > --- > include/trace/events/rcu.h | 25 +++++++++++++++++++++++++ > kernel/rcu/rcu_segcblist.c | 34 ++++++++++++++++++++++++++++++++++ > kernel/rcu/rcu_segcblist.h | 5 +++++ > kernel/rcu/tree.c | 9 +++++++++ > 4 files changed, 73 insertions(+) > > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h > index 155b5cb43cfd..7b84df3c95df 100644 > --- a/include/trace/events/rcu.h > +++ b/include/trace/events/rcu.h > @@ -505,6 +505,31 @@ 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)
Use RCU_CBLIST_NSEGS in place of 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 > * kvfree() 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 0e6d19bd3de9..df0f31e30947 100644 > --- a/kernel/rcu/rcu_segcblist.c > +++ b/kernel/rcu/rcu_segcblist.c > @@ -13,6 +13,7 @@ > #include <linux/rcupdate.h> > > #include "rcu_segcblist.h" > +#include "rcu.h" > > /* Initialize simple callback list. */ > void rcu_cblist_init(struct rcu_cblist *rclp) > @@ -343,6 +344,39 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp, > rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0); > } > > +/* > + * 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
N is number of segments?
> + * tracing code which is usually disabled in production. > + */ > +#ifdef CONFIG_RCU_TRACE > +static void rcu_segcblist_countseq(struct rcu_segcblist *rsclp, > + int cbcount[RCU_CBLIST_NSEGS], > + unsigned long gpseq[RCU_CBLIST_NSEGS]) > +{ > + int i; > + > + for (i = 0; i < RCU_CBLIST_NSEGS; i++) > + cbcount[i] = 0; > +
What is the reason for initializing to 0?
> + for (i = 0; i < RCU_CBLIST_NSEGS; i++) { > + cbcount[i] = rcu_segcblist_get_seglen(rsclp, i); > + gpseq[i] = rsclp->gp_seq[i]; > + } > +} > + > +void trace_rcu_segcb_list(struct rcu_segcblist *rsclp, char *context) > +{ > + int cbs[RCU_CBLIST_NSEGS]; > + unsigned long gps[RCU_CBLIST_NSEGS]; > + > + rcu_segcblist_countseq(rsclp, cbs, gps); > + > + trace_rcu_segcb(context, cbs, gps); > +} > +#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 3e0eb1056ae9..15c10d30f88c 100644 > --- a/kernel/rcu/rcu_segcblist.h > +++ b/kernel/rcu/rcu_segcblist.h > @@ -103,3 +103,8 @@ 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 trace_rcu_segcb_list(struct rcu_segcblist *rsclp, char *context); > +#else > +#define trace_rcu_segcb_list(...) > +#endif > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 50af465729f4..e3381ff67fc6 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1492,6 +1492,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) > if (!rcu_segcblist_pend_cbs(&rdp->cblist)) > return false; > > + trace_rcu_segcb_list(&rdp->cblist, "SegCbPreAcc");
Use TPS("SegCbPreAcc") ?
Thanks Neeraj
> + > /* > * Callbacks are often registered with incomplete grace-period > * information. Something about the fact that getting exact > @@ -1512,6 +1514,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) > else > trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB")); > > + trace_rcu_segcb_list(&rdp->cblist, "SegCbPostAcc"); > + > return ret; > } > > @@ -2469,6 +2473,9 @@ 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); > + > + trace_rcu_segcb_list(&rdp->cblist, "SegCbDequeued"); > + > for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) { > rcu_callback_t f; > > @@ -2982,6 +2989,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func) > trace_rcu_callback(rcu_state.name, head, > rcu_segcblist_n_cbs(&rdp->cblist)); > > + trace_rcu_segcb_list(&rdp->cblist, "SegCBQueued"); > + > /* Go handle any RCU core processing required. */ > if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) && > unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) { >
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