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McKenney" , rcu@vger.kernel.org, Steven Rostedt , "Uladzislau Rezki (Sony)" Subject: Re: [PATCH v6 3/4] rcu/trace: Add tracing for how segcb list changes Message-ID: <20201015000503.GB4025103@google.com> References: <20200923152211.2403352-1-joel@joelfernandes.org> <20200923152211.2403352-4-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 14, 2020 at 08:52:17PM +0530, Neeraj Upadhyay wrote: > > > 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) > > --- > > 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 ? Done. > > + ), > > + > > + 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 > > #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? Yes, will fix. > > + * 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? You are right, not needed. I'll remove. > > + 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") ? Fixed, thanks! thanks, - Joel > > > 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))) { > > > > -- > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of > the Code Aurora Forum, hosted by The Linux Foundation