From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
This commit replaces the hard-coded seven-jiffy cond_resched()
quiescent-state solicitation delay with a delay that can be set via
the rcutree.jiffies_till_cond_resched_qs boot parameter, or of course
via sysfs. This change was inspired in part by Dave Hansen's time-based
approach: https://lkml.org/lkml/2014/6/17/746.
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Josh Triplett <j...@joshtriplett.org>
Cc: Andi Kleen <a...@linux.intel.com>
Cc: Christoph Lameter <c...@gentwo.org>
Cc: Mike Galbraith <umgwanakikb...@gmail.com>
Cc: Eric Dumazet <eric.duma...@gmail.com>
---
Documentation/kernel-parameters.txt | 7 +++++++
kernel/rcu/tree.c | 24 ++++++++++++++++++++++--
2 files changed, 29 insertions(+), 2 deletions(-)
diff --git a/Documentation/kernel-parameters.txt
b/Documentation/kernel-parameters.txt
index 6eaa9cdb7094..1b553596e933 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2785,6 +2785,13 @@ bytes respectively. Such letter suffixes can also be
entirely omitted.
leaf rcu_node structure. Useful for very large
systems.
+ rcutree.jiffies_till_cond_resched_qs= [KNL]
+ Set required age in jiffies for a given
+ grace period before RCU starts soliciting
+ quiescent-state help from cond_resched_rcu_qs(),
+ and, if CONFIG_RCU_COND_RESCHED_QS=y, also from
+ cond_resched() itself.
+
rcutree.jiffies_till_first_fqs= [KNL]
Set delay from grace-period initialization to
first attempt to force quiescent states.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8d1f45b41433..d88f6a10d971 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -295,6 +295,14 @@ static ulong jiffies_till_next_fqs = ULONG_MAX;
module_param(jiffies_till_first_fqs, ulong, 0644);
module_param(jiffies_till_next_fqs, ulong, 0644);
+/*
+ * How long the grace period must be before we start recruiting
+ * quiescent-state help from cond_resched_rcu_qs() and, if
+ * CONFIG_RCU_COND_RESCHED_QS=y, also from cond_resched() itself.
+ */
+static ulong jiffies_till_cond_resched_qs = 10;
+module_param(jiffies_till_cond_resched_qs, ulong, 0644);
+
static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
struct rcu_data *rdp);
static void force_qs_rnp(struct rcu_state *rsp,
@@ -951,9 +959,21 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
* even context-switching back and forth between a pair of
* in-kernel CPU-bound tasks cannot advance grace periods.
* So if the grace period is old enough, make the CPU pay attention.
+ * Note that the unsynchronized assignments to the per-CPU
+ * rcu_cond_resched_mask variable are safe. Yes, setting of
+ * bits can be lost, but they will be set again on the next
+ * force-quiescent-state pass. So lost bit sets do not result
+ * in incorrect behavior, merely in a grace period lasting
+ * a few jiffies longer than it might otherwise. Because
+ * there are at most four threads involved, and because the
+ * updates are only once every few jiffies, the probability of
+ * lossage (and thus of slight grace-period extension) is
+ * quite low.
*/
- if (ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + 7)) {
- rcrmp = &per_cpu(rcu_cond_resched_mask, rdp->cpu);
+ rcrmp = &per_cpu(rcu_cond_resched_mask, rdp->cpu);
+ if (ULONG_CMP_GE(jiffies,
+ rdp->rsp->gp_start + jiffies_till_cond_resched_qs) &&
+ !(ACCESS_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
ACCESS_ONCE(rdp->cond_resched_completed) =
ACCESS_ONCE(rdp->mynode->completed);
smp_mb(); /* ->cond_resched_completed before *rcrmp. */
--
1.8.1.5
--
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