The following commit has been merged into the core/rcu branch of tip: Commit-ID: 5648d6591230c811972543ff146ce969babdd732 Gitweb: https://git.kernel.org/tip/5648d6591230c811972543ff146ce969babdd732 Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Tue, 21 Jan 2020 12:30:22 -08:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Thu, 20 Feb 2020 15:58:23 -08:00 rcu: Don't flag non-starting GPs before GP kthread is running Currently rcu_check_gp_start_stall() complains if a grace period takes too long to start, where "too long" is roughly one RCU CPU stall-warning interval. This has worked well, but there are some debugging Kconfig options (such as CONFIG_EFI_PGT_DUMP=y) that can make booting take a very long time, so much so that the stall-warning interval has expired before RCU's grace-period kthread has even been spawned. This commit therefore resets the rcu_state.gp_req_activity and rcu_state.gp_activity timestamps just before the grace-period kthread is spawned, and modifies the checks and adds ordering to ensure that if rcu_check_gp_start_stall() sees that the grace-period kthread has been spawned, that it will also see the resets applied to the rcu_state.gp_req_activity and rcu_state.gp_activity timestamps. Reported-by: Qian Cai <cai@xxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> [ paulmck: Fix whitespace issues reported by Qian Cai. ] Tested-by: Qian Cai <cai@xxxxxx> [ paulmck: Simplify grace-period wakeup check per Steve Rostedt feedback. ] --- kernel/rcu/tree.c | 28 ++++++++++++++++------------ kernel/rcu/tree_stall.h | 7 ++++--- 2 files changed, 20 insertions(+), 15 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 62383ce..4a4a975 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1202,7 +1202,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp, trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot")); WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT); WRITE_ONCE(rcu_state.gp_req_activity, jiffies); - if (!rcu_state.gp_kthread) { + if (!READ_ONCE(rcu_state.gp_kthread)) { trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread")); goto unlock_out; } @@ -1237,12 +1237,13 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp) } /* - * Awaken the grace-period kthread. Don't do a self-awaken (unless in - * an interrupt or softirq handler), and don't bother awakening when there - * is nothing for the grace-period kthread to do (as in several CPUs raced - * to awaken, and we lost), and finally don't try to awaken a kthread that - * has not yet been created. If all those checks are passed, track some - * debug information and awaken. + * Awaken the grace-period kthread. Don't do a self-awaken (unless in an + * interrupt or softirq handler, in which case we just might immediately + * sleep upon return, resulting in a grace-period hang), and don't bother + * awakening when there is nothing for the grace-period kthread to do + * (as in several CPUs raced to awaken, we lost), and finally don't try + * to awaken a kthread that has not yet been created. If all those checks + * are passed, track some debug information and awaken. * * So why do the self-wakeup when in an interrupt or softirq handler * in the grace-period kthread's context? Because the kthread might have @@ -1252,10 +1253,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp) */ static void rcu_gp_kthread_wake(void) { - if ((current == rcu_state.gp_kthread && - !in_irq() && !in_serving_softirq()) || - !READ_ONCE(rcu_state.gp_flags) || - !rcu_state.gp_kthread) + struct task_struct *t = READ_ONCE(rcu_state.gp_kthread); + + if ((current == t && !in_irq() && !in_serving_softirq()) || + !READ_ONCE(rcu_state.gp_flags) || !t) return; WRITE_ONCE(rcu_state.gp_wake_time, jiffies); WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq)); @@ -3554,7 +3555,10 @@ static int __init rcu_spawn_gp_kthread(void) } rnp = rcu_get_root(); raw_spin_lock_irqsave_rcu_node(rnp, flags); - rcu_state.gp_kthread = t; + WRITE_ONCE(rcu_state.gp_activity, jiffies); + WRITE_ONCE(rcu_state.gp_req_activity, jiffies); + // Reset .gp_activity and .gp_req_activity before setting .gp_kthread. + smp_store_release(&rcu_state.gp_kthread, t); /* ^^^ */ raw_spin_unlock_irqrestore_rcu_node(rnp, flags); wake_up_process(t); rcu_spawn_nocb_kthreads(); diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 488b71d..16ad7ad 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -578,6 +578,7 @@ void show_rcu_gp_kthreads(void) unsigned long jw; struct rcu_data *rdp; struct rcu_node *rnp; + struct task_struct *t = READ_ONCE(rcu_state.gp_kthread); j = jiffies; ja = j - READ_ONCE(rcu_state.gp_activity); @@ -585,8 +586,7 @@ void show_rcu_gp_kthreads(void) jw = j - READ_ONCE(rcu_state.gp_wake_time); pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n", rcu_state.name, gp_state_getname(rcu_state.gp_state), - rcu_state.gp_state, - rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL, + rcu_state.gp_state, t ? t->state : 0x1ffffL, ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq), (long)READ_ONCE(rcu_state.gp_seq), (long)READ_ONCE(rcu_get_root()->gp_seq_needed), @@ -633,7 +633,8 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp, if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() || ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq), - READ_ONCE(rnp_root->gp_seq_needed))) + READ_ONCE(rnp_root->gp_seq_needed)) || + !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread. return; j = jiffies; /* Expensive access, and in common case don't get here. */ if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
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