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rcu: Remove restrictions on no-CBs CPUs 

Currently, CPU 0 is constrained to not be a no-CBs CPU, and furthermore
at least one no-CBs CPU must remain online at any given time.  These
restrictions are problematic in some situations, such as cases where
all CPUs must run a real-time workload that needs to be insulated from
OS jitter and latencies due to RCU callback invocation.  This commit
therefore provides no-CBs CPUs a (very crude and energy-inefficient)
way to start and to wait for grace periods independently of the normal
RCU callback mechanisms.  This approach allows any or all of the CPUs to
be designated as no-CBs CPUs, and allows any proper subset of the CPUs
(whether no-CBs CPUs or not) to be offlined.

This commit also provides a fix for a locking bug spotted by Xie
ChanglongX <changlongx.xie@intel.com>.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2013-01-07 13:37:42 -08:00 committed by Paul E. McKenney
parent f6161aa153
commit 34ed62461a
4 changed files with 58 additions and 130 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
init/Kconfig
View File

@ -655,7 +655,7 @@ config RCU_BOOST_DELAY
Accept the default if unsure.
config RCU_NOCB_CPU
bool "Offload RCU callback processing from boot-selected CPUs"
bool "Offload RCU callback processing from boot-selected CPUs (EXPERIMENTAL"
depends on TREE_RCU || TREE_PREEMPT_RCU
default n
help
@ -673,7 +673,7 @@ config RCU_NOCB_CPU
callback, and (2) affinity or cgroups can be used to force
the kthreads to run on whatever set of CPUs is desired.
Say Y here if you want reduced OS jitter on selected CPUs.
Say Y here if you want to help to debug reduced OS jitter.
Say N here if you are unsure.
endmenu # "RCU Subsystem"

12
kernel/rcutree.c
View File

@ -310,6 +310,8 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
if (rcu_gp_in_progress(rsp))
return 0; /* No, a grace period is already in progress. */
if (rcu_nocb_needs_gp(rdp))
return 1; /* Yes, a no-CBs CPU needs one. */
if (!rdp->nxttail[RCU_NEXT_TAIL])
return 0; /* No, this is a no-CBs (or offline) CPU. */
if (*rdp->nxttail[RCU_NEXT_READY_TAIL])
@ -1035,10 +1037,11 @@ static void init_callback_list(struct rcu_data *rdp)
{
int i;
if (init_nocb_callback_list(rdp))
return;
rdp->nxtlist = NULL;
for (i = 0; i < RCU_NEXT_SIZE; i++)
rdp->nxttail[i] = &rdp->nxtlist;
init_nocb_callback_list(rdp);
}
/*
@ -2909,7 +2912,6 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;
struct rcu_state *rsp;
int ret = NOTIFY_OK;
trace_rcu_utilization("Start CPU hotplug");
switch (action) {
@ -2923,10 +2925,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
rcu_boost_kthread_setaffinity(rnp, -1);
break;
case CPU_DOWN_PREPARE:
if (nocb_cpu_expendable(cpu))
rcu_boost_kthread_setaffinity(rnp, cpu);
else
ret = NOTIFY_BAD;
break;
case CPU_DYING:
case CPU_DYING_FROZEN:
@ -2950,7 +2949,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
break;
}
trace_rcu_utilization("End CPU hotplug");
return ret;
return NOTIFY_OK;
}
/*
@ -3170,7 +3169,6 @@ void __init rcu_init(void)
rcu_init_one(&rcu_sched_state, &rcu_sched_data);
rcu_init_one(&rcu_bh_state, &rcu_bh_data);
__rcu_init_preempt();
rcu_init_nocb();
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
/*

12
kernel/rcutree.h
View File

@ -326,6 +326,7 @@ struct rcu_data {
int nocb_p_count_lazy; /* (approximate). */
wait_queue_head_t nocb_wq; /* For nocb kthreads to sleep on. */
struct task_struct *nocb_kthread;
bool nocb_needs_gp;
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
int cpu;
@ -375,12 +376,6 @@ struct rcu_state {
struct rcu_data __percpu *rda; /* pointer of percu rcu_data. */
void (*call)(struct rcu_head *head, /* call_rcu() flavor. */
void (*func)(struct rcu_head *head));
#ifdef CONFIG_RCU_NOCB_CPU
void (*call_remote)(struct rcu_head *head,
void (*func)(struct rcu_head *head));
/* call_rcu() flavor, but for */
/* placing on remote CPU. */
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
/* The following fields are guarded by the root rcu_node's lock. */
@ -529,16 +524,15 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
static void print_cpu_stall_info_end(void);
static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static void increment_cpu_stall_ticks(void);
static int rcu_nocb_needs_gp(struct rcu_data *rdp);
static bool is_nocb_cpu(int cpu);
static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
bool lazy);
static bool rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
struct rcu_data *rdp);
static bool nocb_cpu_expendable(int cpu);
static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
static void init_nocb_callback_list(struct rcu_data *rdp);
static void __init rcu_init_nocb(void);
static bool init_nocb_callback_list(struct rcu_data *rdp);
#endif /* #ifndef RCU_TREE_NONCORE */

158
kernel/rcutree_plugin.h
View File

@ -86,10 +86,6 @@ static void __init rcu_bootup_announce_oddness(void)
printk(KERN_INFO "\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
#ifdef CONFIG_RCU_NOCB_CPU
if (have_rcu_nocb_mask) {
if (cpumask_test_cpu(0, rcu_nocb_mask)) {
cpumask_clear_cpu(0, rcu_nocb_mask);
pr_info("\tCPU 0: illegal no-CBs CPU (cleared).\n");
}
cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
pr_info("\tExperimental no-CBs CPUs: %s.\n", nocb_buf);
if (rcu_nocb_poll)
@ -2165,6 +2161,14 @@ static int __init parse_rcu_nocb_poll(char *arg)
}
early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
/*
* Does this CPU needs a grace period due to offloaded callbacks?
*/
static int rcu_nocb_needs_gp(struct rcu_data *rdp)
{
return rdp->nocb_needs_gp;
}
/* Is the specified CPU a no-CPUs CPU? */
static bool is_nocb_cpu(int cpu)
{
@ -2265,95 +2269,39 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
}
/*
* There must be at least one non-no-CBs CPU in operation at any given
* time, because no-CBs CPUs are not capable of initiating grace periods
* independently. This function therefore complains if the specified
* CPU is the last non-no-CBs CPU, allowing the CPU-hotplug system to
* avoid offlining the last such CPU. (Recursion is a wonderful thing,
* but you have to have a base case!)
* If necessary, kick off a new grace period, and either way wait
* for a subsequent grace period to complete.
*/
static bool nocb_cpu_expendable(int cpu)
static void rcu_nocb_wait_gp(struct rcu_data *rdp)
{
cpumask_var_t non_nocb_cpus;
int ret;
unsigned long c;
unsigned long flags;
unsigned long j;
struct rcu_node *rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
c = rnp->completed + 2;
rdp->nocb_needs_gp = true;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
/*
* If there are no no-CB CPUs or if this CPU is not a no-CB CPU,
* then offlining this CPU is harmless. Let it happen.
* Wait for the grace period. Do so interruptibly to avoid messing
* up the load average.
*/
if (!have_rcu_nocb_mask || is_nocb_cpu(cpu))
return 1;
/* If no memory, play it safe and keep the CPU around. */
if (!alloc_cpumask_var(&non_nocb_cpus, GFP_NOIO))
return 0;
cpumask_andnot(non_nocb_cpus, cpu_online_mask, rcu_nocb_mask);
cpumask_clear_cpu(cpu, non_nocb_cpus);
ret = !cpumask_empty(non_nocb_cpus);
free_cpumask_var(non_nocb_cpus);
return ret;
}
/*
* Helper structure for remote registry of RCU callbacks.
* This is needed for when a no-CBs CPU needs to start a grace period.
* If it just invokes call_rcu(), the resulting callback will be queued,
* which can result in deadlock.
*/
struct rcu_head_remote {
struct rcu_head *rhp;
call_rcu_func_t *crf;
void (*func)(struct rcu_head *rhp);
};
/*
* Register a callback as specified by the rcu_head_remote struct.
* This function is intended to be invoked via smp_call_function_single().
*/
static void call_rcu_local(void *arg)
{
struct rcu_head_remote *rhrp =
container_of(arg, struct rcu_head_remote, rhp);
rhrp->crf(rhrp->rhp, rhrp->func);
}
/*
* Set up an rcu_head_remote structure and the invoke call_rcu_local()
* on CPU 0 (which is guaranteed to be a non-no-CBs CPU) via
* smp_call_function_single().
*/
static void invoke_crf_remote(struct rcu_head *rhp,
void (*func)(struct rcu_head *rhp),
call_rcu_func_t crf)
{
struct rcu_head_remote rhr;
rhr.rhp = rhp;
rhr.crf = crf;
rhr.func = func;
smp_call_function_single(0, call_rcu_local, &rhr, 1);
}
/*
* Helper functions to be passed to wait_rcu_gp(), each of which
* invokes invoke_crf_remote() to register a callback appropriately.
*/
static void __maybe_unused
call_rcu_preempt_remote(struct rcu_head *rhp,
void (*func)(struct rcu_head *rhp))
{
invoke_crf_remote(rhp, func, call_rcu);
}
static void call_rcu_bh_remote(struct rcu_head *rhp,
void (*func)(struct rcu_head *rhp))
{
invoke_crf_remote(rhp, func, call_rcu_bh);
}
static void call_rcu_sched_remote(struct rcu_head *rhp,
void (*func)(struct rcu_head *rhp))
{
invoke_crf_remote(rhp, func, call_rcu_sched);
for (;;) {
j = jiffies;
schedule_timeout_interruptible(2);
raw_spin_lock_irqsave(&rnp->lock, flags);
if (ULONG_CMP_GE(rnp->completed, c)) {
rdp->nocb_needs_gp = false;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
break;
}
if (j == jiffies)
flush_signals(current);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
smp_mb(); /* Ensure that CB invocation happens after GP end. */
}
/*
@ -2390,7 +2338,7 @@ static int rcu_nocb_kthread(void *arg)
cl = atomic_long_xchg(&rdp->nocb_q_count_lazy, 0);
ACCESS_ONCE(rdp->nocb_p_count) += c;
ACCESS_ONCE(rdp->nocb_p_count_lazy) += cl;
wait_rcu_gp(rdp->rsp->call_remote);
rcu_nocb_wait_gp(rdp);
/* Each pass through the following loop invokes a callback. */
trace_rcu_batch_start(rdp->rsp->name, cl, c, -1);
@ -2443,26 +2391,22 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
}
/* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
static void init_nocb_callback_list(struct rcu_data *rdp)
static bool init_nocb_callback_list(struct rcu_data *rdp)
{
if (rcu_nocb_mask == NULL ||
!cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
return;
return false;
rdp->nxttail[RCU_NEXT_TAIL] = NULL;
}
/* Initialize the ->call_remote fields in the rcu_state structures. */
static void __init rcu_init_nocb(void)
{
#ifdef CONFIG_PREEMPT_RCU
rcu_preempt_state.call_remote = call_rcu_preempt_remote;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
rcu_bh_state.call_remote = call_rcu_bh_remote;
rcu_sched_state.call_remote = call_rcu_sched_remote;
return true;
}
#else /* #ifdef CONFIG_RCU_NOCB_CPU */
static int rcu_nocb_needs_gp(struct rcu_data *rdp)
{
return 0;
}
static bool is_nocb_cpu(int cpu)
{
return false;
@ -2480,11 +2424,6 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
return 0;
}
static bool nocb_cpu_expendable(int cpu)
{
return 1;
}
static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
}
@ -2493,12 +2432,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
{
}
static void init_nocb_callback_list(struct rcu_data *rdp)
{
}
static void __init rcu_init_nocb(void)
static bool init_nocb_callback_list(struct rcu_data *rdp)
{
return false;
}
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */