Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH tip/core/rcu 01/15] sched,rcu: Make cond_resched() provide RCU quiescent state | Date | Mon, 24 Jul 2017 14:44:30 -0700 |
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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 <mhocko@kernel.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> --- include/linux/sched.h | 3 ++- kernel/rcu/tree.c | 23 ++++------------------- kernel/sched/core.c | 1 + 3 files changed, 7 insertions(+), 20 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index 2ba9ec93423f..12f326aa5871 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1508,10 +1508,11 @@ 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 51d4c3acf32d..ab4c2cea208d 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -537,8 +537,8 @@ 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 / 16; +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); @@ -1230,7 +1230,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp) unsigned long jtsq; bool *rnhqp; bool *ruqp; - unsigned long rjtsc; struct rcu_node *rnp; /* @@ -1247,23 +1246,13 @@ 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) && @@ -1271,7 +1260,7 @@ 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); } @@ -1299,10 +1288,6 @@ 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) && diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 17c667b427b4..9433633012ba 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4808,6 +4808,7 @@ int __sched _cond_resched(void) preempt_schedule_common(); return 1; } + rcu_all_qs(); return 0; } EXPORT_SYMBOL(_cond_resched); -- 2.5.2
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