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McKenney" , rcu@vger.kernel.org, Steven Rostedt , "Uladzislau Rezki (Sony)" Subject: Re: [PATCH v6 1/4] rcu/tree: Make rcu_do_batch count how many callbacks were executed Message-ID: <20201014152326.GA4022964@google.com> References: <20200923152211.2403352-1-joel@joelfernandes.org> <20200923152211.2403352-2-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 08:36:16PM +0530, Neeraj Upadhyay wrote: > > > On 9/23/2020 8:52 PM, Joel Fernandes (Google) wrote: > > Currently, rcu_do_batch() depends on the unsegmented callback list's len field > > to know how many CBs are executed. This fields counts down from 0 as CBs are > > dequeued. It is possible that all CBs could not be run because of reaching > > limits in which case the remaining unexecuted callbacks are requeued in the > > CPU's segcblist. > > > > The number of callbacks that were not requeued are then the negative count (how > > many CBs were run) stored in the rcl->len which has been counting down on every > > dequeue. This negative count is then added to the per-cpu segmented callback > > list's to correct its count. > > > > Such a design works against future efforts to track the length of each segment > > of the segmented callback list. The reason is because > > rcu_segcblist_extract_done_cbs() will be populating the unsegmented callback > > list's length field (rcl->len) during extraction. > > > > Also, the design of counting down from 0 is confusing and error-prone IMHO. > > > > This commit therefore explicitly counts have many callbacks were executed in > > rcu_do_batch() itself, and uses that to update the per-CPU segcb list's ->len > > field, without relying on the negativity of rcl->len. > > > > Signed-off-by: Joel Fernandes (Google) > > --- > > kernel/rcu/rcu_segcblist.c | 2 +- > > kernel/rcu/rcu_segcblist.h | 1 + > > kernel/rcu/tree.c | 9 ++++----- > > 3 files changed, 6 insertions(+), 6 deletions(-) > > > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c > > index 2d2a6b6b9dfb..bb246d8c6ef1 100644 > > --- a/kernel/rcu/rcu_segcblist.c > > +++ b/kernel/rcu/rcu_segcblist.c > > @@ -95,7 +95,7 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v) > > * This increase is fully ordered with respect to the callers accesses > > * both before and after. > > */ > > -static void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v) > > +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v) > > { > > #ifdef CONFIG_RCU_NOCB_CPU > > smp_mb__before_atomic(); /* Up to the caller! */ > > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h > > index 5c293afc07b8..b90725f81d77 100644 > > --- a/kernel/rcu/rcu_segcblist.h > > +++ b/kernel/rcu/rcu_segcblist.h > > @@ -76,6 +76,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg) > > } > > void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp); > > +void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v); > > void rcu_segcblist_init(struct rcu_segcblist *rsclp); > > void rcu_segcblist_disable(struct rcu_segcblist *rsclp); > > void rcu_segcblist_offload(struct rcu_segcblist *rsclp); > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 7623128d0020..50af465729f4 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -2427,7 +2427,7 @@ static void rcu_do_batch(struct rcu_data *rdp) > > rcu_segcblist_is_offloaded(&rdp->cblist); > > struct rcu_head *rhp; > > struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl); > > - long bl, count; > > + long bl, count = 0; > > long pending, tlimit = 0; > > /* If no callbacks are ready, just return. */ > > @@ -2472,6 +2472,7 @@ static void rcu_do_batch(struct rcu_data *rdp) > > for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) { > > rcu_callback_t f; > > + count++; > > debug_rcu_head_unqueue(rhp); > > rcu_lock_acquire(&rcu_callback_map); > > @@ -2485,9 +2486,8 @@ static void rcu_do_batch(struct rcu_data *rdp) > > /* > > * Stop only if limit reached and CPU has something to do. > > - * Note: The rcl structure counts down from zero. > > */ > > - if (-rcl.len >= bl && !offloaded && > > + if (count >= bl && !offloaded && > > (need_resched() || > > (!is_idle_task(current) && !rcu_is_callbacks_kthread()))) > > break; > > Update below usage of -rcl.len also? > > if (likely((-rcl.len & 31) || local_clock() < tlimit)) Yes, you are right. I need to change that as well, will do. Thanks! thanks, - Joel