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Subject[PATCH v2 tip/core/rcu 11/39] rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions
Date
The rcu_all_qs() and rcu_note_context_switch() do a series of checks,
taking various actions to supply RCU with quiescent states, depending
on the outcomes of the various checks. This is a bit much for scheduling
fastpaths, so this commit creates a separate ->rcu_urgent_qs field in
the rcu_dynticks structure that acts as a global guard for these checks.
Thus, in the common case, rcu_all_qs() and rcu_note_context_switch()
check the ->rcu_urgent_qs field, find it false, and simply return.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
.../Design/Data-Structures/Data-Structures.html | 11 ++++++-
kernel/rcu/tree.c | 38 ++++++++++++++--------
kernel/rcu/tree.h | 3 +-
kernel/rcu/tree_exp.h | 2 ++
kernel/rcu/tree_plugin.h | 8 +++--
5 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index e4bf20a68fa3..4dec89097559 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1106,6 +1106,7 @@ Its fields are as follows:
3 atomic_t dynticks;
4 bool rcu_need_heavy_qs;
5 unsigned long rcu_qs_ctr;
+ 6 bool rcu_urgent_qs;
</pre>

<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
@@ -1131,12 +1132,20 @@ it is willing to call for heavy-weight dyntick-counter operations.
This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
code, which provide a momentary idle sojourn in response.

-</p><p>Finally the <tt>-&gt;rcu_qs_ctr</tt> field is used to record
+</p><p>The <tt>-&gt;rcu_qs_ctr</tt> field is used to record
quiescent states from <tt>cond_resched()</tt>.
Because <tt>cond_resched()</tt> can execute quite frequently, this
must be quite lightweight, as in a non-atomic increment of this
per-CPU field.

+</p><p>Finally, the <tt>-&gt;rcu_urgent_qs</tt> field is used to record
+the fact that the RCU core code would really like to see a quiescent
+state from the corresponding CPU, with the various other fields indicating
+just how badly RCU wants this quiescent state.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which, if nothing else, non-atomically increment <tt>-&gt;rcu_qs_ctr</tt>
+in response.
+
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3c62ea06edb3..aa81cc6c365e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -464,8 +464,14 @@ void rcu_note_context_switch(void)
trace_rcu_utilization(TPS("Start context switch"));
rcu_sched_qs();
rcu_preempt_note_context_switch();
+ /* Load rcu_urgent_qs before other flags. */
+ if (!smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)))
+ goto out;
+ this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
if (unlikely(raw_cpu_read(rcu_dynticks.rcu_need_heavy_qs)))
rcu_momentary_dyntick_idle();
+ this_cpu_inc(rcu_dynticks.rcu_qs_ctr);
+out:
trace_rcu_utilization(TPS("End context switch"));
barrier(); /* Avoid RCU read-side critical sections leaking up. */
}
@@ -488,29 +494,26 @@ void rcu_all_qs(void)
{
unsigned long flags;

+ if (!raw_cpu_read(rcu_dynticks.rcu_urgent_qs))
+ return;
+ preempt_disable();
+ /* Load rcu_urgent_qs before other flags. */
+ if (!smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
+ preempt_enable();
+ return;
+ }
+ this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
barrier(); /* Avoid RCU read-side critical sections leaking down. */
if (unlikely(raw_cpu_read(rcu_dynticks.rcu_need_heavy_qs))) {
local_irq_save(flags);
rcu_momentary_dyntick_idle();
local_irq_restore(flags);
}
- if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))) {
- /*
- * Yes, we just checked a per-CPU variable with preemption
- * enabled, so we might be migrated to some other CPU at
- * this point. That is OK because in that case, the
- * migration will supply the needed quiescent state.
- * We might end up needlessly disabling preemption and
- * invoking rcu_sched_qs() on the destination CPU, but
- * the probability and cost are both quite low, so this
- * should not be a problem in practice.
- */
- preempt_disable();
+ if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp)))
rcu_sched_qs();
- preempt_enable();
- }
this_cpu_inc(rcu_dynticks.rcu_qs_ctr);
barrier(); /* Avoid RCU read-side critical sections leaking up. */
+ preempt_enable();
}
EXPORT_SYMBOL_GPL(rcu_all_qs);

@@ -1246,6 +1249,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
{
unsigned long jtsq;
bool *rnhqp;
+ bool *ruqp;
unsigned long rjtsc;
struct rcu_node *rnp;

@@ -1281,11 +1285,15 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
* might not be the case for nohz_full CPUs looping in the kernel.
*/
rnp = rdp->mynode;
+ ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_dynticks.rcu_qs_ctr, rdp->cpu) &&
READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
return 1;
+ } else {
+ /* Load rcu_qs_ctr before store to rcu_urgent_qs. */
+ smp_store_release(ruqp, true);
}

/* Check for the CPU being offline. */
@@ -1321,6 +1329,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
(time_after(jiffies, rdp->rsp->gp_start + jtsq) ||
time_after(jiffies, rdp->rsp->jiffies_resched))) {
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. */
}

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index b212cd0f22c7..d2f276fc2edc 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -113,8 +113,9 @@ struct rcu_dynticks {
/* Process level is worth LLONG_MAX/2. */
int dynticks_nmi_nesting; /* Track NMI nesting level. */
atomic_t dynticks; /* Even value for idle, else odd. */
- bool rcu_need_heavy_qs; /* GP old, need heavy quiescent state. */
+ bool rcu_need_heavy_qs; /* GP old, need heavy quiescent state. */
unsigned long rcu_qs_ctr; /* Light universal quiescent state ctr. */
+ bool rcu_urgent_qs; /* GP old need light quiescent state. */
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
long long dynticks_idle_nesting;
/* irq/process nesting level from idle. */
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index a7b639ccd46e..a1f52bbe9db6 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -331,6 +331,8 @@ static void sync_sched_exp_handler(void *data)
return;
}
__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
+ /* Store .exp before .rcu_urgent_qs. */
+ smp_store_release(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs), true);
resched_cpu(smp_processor_id());
}

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0a62a8f1caac..621296a6694b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1860,7 +1860,9 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
TPS("WakeEmpty"));
} else {
- rdp->nocb_defer_wakeup = RCU_NOGP_WAKE;
+ WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOGP_WAKE);
+ /* Store ->nocb_defer_wakeup before ->rcu_urgent_qs. */
+ smp_store_release(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs), true);
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
TPS("WakeEmptyIsDeferred"));
}
@@ -1872,7 +1874,9 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
TPS("WakeOvf"));
} else {
- rdp->nocb_defer_wakeup = RCU_NOGP_WAKE_FORCE;
+ WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOGP_WAKE_FORCE);
+ /* Store ->nocb_defer_wakeup before ->rcu_urgent_qs. */
+ smp_store_release(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs), true);
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
TPS("WakeOvfIsDeferred"));
}
--
2.5.2
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