The following commit has been merged into the core/rcu branch of tip: Commit-ID: 53922270d21de707a1a0ffaf1e07644e77fcb8db Gitweb: https://git.kernel.org/tip/53922270d21de707a1a0ffaf1e07644e77fcb8db Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> AuthorDate: Thu, 18 Jun 2020 16:29:49 -04:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Thu, 03 Sep 2020 09:39:59 -07:00 rcu/segcblist: Prevent useless GP start if no CBs to accelerate The rcu_segcblist_accelerate() function returns true iff it is necessary to request another grace period. A tracing session showed that this function unnecessarily requests grace periods. For example, consider the following sequence of events: 1. Callbacks are queued only on the NEXT segment of CPU A's callback list. 2. CPU A runs RCU_SOFTIRQ, accelerating these callbacks from NEXT to WAIT. 3. Thus rcu_segcblist_accelerate() returns true, requesting grace period N. 4. RCU's grace-period kthread wakes up on CPU B and starts grace period N. 4. CPU A notices the new grace period and invokes RCU_SOFTIRQ. 5. CPU A's RCU_SOFTIRQ again invokes rcu_segcblist_accelerate(), but there are no new callbacks. However, rcu_segcblist_accelerate() nevertheless (uselessly) requests a new grace period N+1. This extra grace period results in additional lock contention and also additional wakeups, all for no good reason. This commit therefore adds a check to rcu_segcblist_accelerate() that prevents the return of true when there are no new callbacks. This change reduces the number of grace periods (GPs) and wakeups in each of eleven five-second rcutorture runs as follows: +----+-------------------+-------------------+ | # | Number of GPs | Number of Wakeups | +====+=========+=========+=========+=========+ | 1 | With | Without | With | Without | +----+---------+---------+---------+---------+ | 2 | 75 | 89 | 113 | 119 | +----+---------+---------+---------+---------+ | 3 | 62 | 91 | 105 | 123 | +----+---------+---------+---------+---------+ | 4 | 60 | 79 | 98 | 110 | +----+---------+---------+---------+---------+ | 5 | 63 | 79 | 99 | 112 | +----+---------+---------+---------+---------+ | 6 | 57 | 89 | 96 | 123 | +----+---------+---------+---------+---------+ | 7 | 64 | 85 | 97 | 118 | +----+---------+---------+---------+---------+ | 8 | 58 | 83 | 98 | 113 | +----+---------+---------+---------+---------+ | 9 | 57 | 77 | 89 | 104 | +----+---------+---------+---------+---------+ | 10 | 66 | 82 | 98 | 119 | +----+---------+---------+---------+---------+ | 11 | 52 | 82 | 83 | 117 | +----+---------+---------+---------+---------+ The reduction in the number of wakeups ranges from 5% to 40%. Cc: urezki@xxxxxxxxx [ paulmck: Rework commit log and comment. ] Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/rcu_segcblist.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index 9a0f661..2d2a6b6 100644 --- a/kernel/rcu/rcu_segcblist.c +++ b/kernel/rcu/rcu_segcblist.c @@ -475,8 +475,16 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq) * Also advance to the oldest segment of callbacks whose * ->gp_seq[] completion is at or after that passed in via "seq", * skipping any empty segments. + * + * Note that segment "i" (and any lower-numbered segments + * containing older callbacks) will be unaffected, and their + * grace-period numbers remain unchanged. For example, if i == + * WAIT_TAIL, then neither WAIT_TAIL nor DONE_TAIL will be touched. + * Instead, the CBs in NEXT_TAIL will be merged with those in + * NEXT_READY_TAIL and the grace-period number of NEXT_READY_TAIL + * would be updated. NEXT_TAIL would then be empty. */ - if (++i >= RCU_NEXT_TAIL) + if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL) return false; /*
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