There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched(). This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead. This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.
Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.
Reported-by: Michal Hocko <mho...@kernel.org>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Peter Zijlstra <pet...@infradead.org>
---
include/linux/sched.h | 3 ++-
kernel/rcu/tree.c | 25 +++++--------------------
kernel/sched/core.c | 1 +
3 files changed, 8 insertions(+), 21 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 26a7df4e558c..59688ef3ea23 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1565,10 +1565,11 @@ static inline int test_tsk_need_resched(struct
task_struct *tsk)
* cond_resched_lock() will drop the spinlock before scheduling,
* cond_resched_softirq() will enable bhs before scheduling.
*/
+void rcu_all_qs(void);
#ifndef CONFIG_PREEMPT
extern int _cond_resched(void);
#else
-static inline int _cond_resched(void) { return 0; }
+static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
#endif
#define cond_resched() ({ \
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0c44c7b42e6d..d3b1d926bb91 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -534,8 +534,8 @@ module_param(rcu_kick_kthreads, bool, 0644);
* How long the grace period must be before we start recruiting
* quiescent-state help from rcu_note_context_switch().
*/
-static ulong jiffies_till_sched_qs = HZ / 20;
-module_param(jiffies_till_sched_qs, ulong, 0644);
+static ulong jiffies_till_sched_qs = HZ / 10;
+module_param(jiffies_till_sched_qs, ulong, 0444);
static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
struct rcu_data *rdp);
@@ -1235,7 +1235,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
unsigned long jtsq;
bool *rnhqp;
bool *ruqp;
- unsigned long rjtsc;
struct rcu_node *rnp;
/*
@@ -1252,23 +1251,13 @@ static int rcu_implicit_dynticks_qs(struct rcu_data
*rdp)
return 1;
}
- /* Compute and saturate jiffies_till_sched_qs. */
- jtsq = jiffies_till_sched_qs;
- rjtsc = rcu_jiffies_till_stall_check();
- if (jtsq > rjtsc / 2) {
- WRITE_ONCE(jiffies_till_sched_qs, rjtsc);
- jtsq = rjtsc / 2;
- } else if (jtsq < 1) {
- WRITE_ONCE(jiffies_till_sched_qs, 1);
- jtsq = 1;
- }
-
/*
* Has this CPU encountered a cond_resched_rcu_qs() since the
* beginning of the grace period? For this to be the case,
* the CPU has to have noticed the current grace period. This
* might not be the case for nohz_full CPUs looping in the kernel.
*/
+ jtsq = jiffies_till_sched_qs;
rnp = rdp->mynode;
ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
@@ -1276,7 +1265,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
return 1;
- } else {
+ } else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
/* Load rcu_qs_ctr before store to rcu_urgent_qs. */
smp_store_release(ruqp, true);
}
@@ -1304,10 +1293,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
* updates are only once every few jiffies, the probability of
* lossage (and thus of slight grace-period extension) is
* quite low.
- *
- * Note that if the jiffies_till_sched_qs boot/sysfs parameter
- * is set too high, we override with half of the RCU CPU stall
- * warning delay.
*/
rnhqp = &per_cpu(rcu_dynticks.rcu_need_heavy_qs, rdp->cpu);
if (!READ_ONCE(*rnhqp) &&
@@ -1316,7 +1301,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
WRITE_ONCE(*rnhqp, true);
/* Store rcu_need_heavy_qs before rcu_urgent_qs. */
smp_store_release(ruqp, true);
- rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
+ rdp->rsp->jiffies_resched += jtsq; /* Re-enable beating. */
}
/*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..904d3ab35c83 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4842,6 +4842,7 @@ int __sched _cond_resched(void)
preempt_schedule_common();
return 1;
}
+ rcu_all_qs();
return 0;
}
EXPORT_SYMBOL(_cond_resched);
--
2.5.2
