From 6193c76aba8ec3cc5f083c35efbab9ed924125f6 Mon Sep 17 00:00:00 2001 From: Paul E. McKenney Date: Mon, 23 Sep 2013 13:57:18 -0700 Subject: rcu: Kick CPU halfway to RCU CPU stall warning When an RCU CPU stall warning occurs, the CPU invokes resched_cpu() on itself. This can help move the grace period forward in some situations, but it would be even better to do this -before- the RCU CPU stall warning. This commit therefore causes resched_cpu() to be called every five jiffies once the system is halfway to an RCU CPU stall warning. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 26 +++++++++++++++++++++++++- kernel/rcu/tree.h | 2 ++ 2 files changed, 27 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index dd081987a8ec..5243ebea0fc1 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -754,6 +754,12 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp, return (rdp->dynticks_snap & 0x1) == 0; } +/* + * This function really isn't for public consumption, but RCU is special in + * that context switches can allow the state machine to make progress. + */ +extern void resched_cpu(int cpu); + /* * Return true if the specified CPU has passed through a quiescent * state by virtue of being in or having passed through an dynticks @@ -812,16 +818,34 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, */ rcu_kick_nohz_cpu(rdp->cpu); + /* + * Alternatively, the CPU might be running in the kernel + * for an extended period of time without a quiescent state. + * Attempt to force the CPU through the scheduler to gain the + * needed quiescent state, but only if the grace period has gone + * on for an uncommonly long time. If there are many stuck CPUs, + * we will beat on the first one until it gets unstuck, then move + * to the next. Only do this for the primary flavor of RCU. + */ + if (rdp->rsp == rcu_state && + ULONG_CMP_GE(ACCESS_ONCE(jiffies), rdp->rsp->jiffies_resched)) { + rdp->rsp->jiffies_resched += 5; + resched_cpu(rdp->cpu); + } + return 0; } static void record_gp_stall_check_time(struct rcu_state *rsp) { unsigned long j = ACCESS_ONCE(jiffies); + unsigned long j1; rsp->gp_start = j; smp_wmb(); /* Record start time before stall time. */ - rsp->jiffies_stall = j + rcu_jiffies_till_stall_check(); + j1 = rcu_jiffies_till_stall_check(); + rsp->jiffies_stall = j + j1; + rsp->jiffies_resched = j + j1 / 2; } /* diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 52be957c9fe2..8e34d8674a4e 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -453,6 +453,8 @@ struct rcu_state { /* but in jiffies. */ unsigned long jiffies_stall; /* Time at which to check */ /* for CPU stalls. */ + unsigned long jiffies_resched; /* Time at which to resched */ + /* a reluctant CPU. */ unsigned long gp_max; /* Maximum GP duration in */ /* jiffies. */ const char *name; /* Name of structure. */ -- cgit v1.2.3-55-g7522