[Xen-changelog] [xen master] xen: RCU: avoid busy waiting until the end of grace period.

[patchbot@xxxxxxx]

commit 2b936ea7b716dc1a13c98550f81752ab053e95c0
Author:     Dario Faggioli <dario.faggioli@xxxxxxxxxx>
AuthorDate: Wed Aug 30 12:06:22 2017 +0100
Commit:     George Dunlap <george.dunlap@xxxxxxxxxx>
CommitDate: Wed Aug 30 12:06:22 2017 +0100

    xen: RCU: avoid busy waiting until the end of grace period.
    
    On the CPU where a callback is queued, cpu_is_haltable()
    returns false (due to rcu_needs_cpu() being itself false).
    That means the CPU would spin inside idle_loop(), continuously
    calling do_softirq(), and, in there, continuously checking
    rcu_pending(), in a tight loop.
    
    Let's instead allow the CPU to really go idle, but make sure,
    by arming a timer, that we periodically check whether the
    grace period has come to an ended. As the period of the
    timer, we pick a value that makes thing look like what
    happens in Linux, with the periodic tick (as this code
    comes from there).
    
    Note that the timer will *only* be armed on CPUs that are
    going idle while having queued RCU callbacks. On CPUs that
    don't, there won't be any timer, and their sleep won't be
    interrupted (and even for CPUs with callbacks, we only
    expect an handful of wakeups at most, but that depends on
    the system load, as much as from other things).
    
    Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
 xen/common/rcupdate.c        | 72 +++++++++++++++++++++++++++++++++++++++++++-
 xen/common/schedule.c        |  2 ++
 xen/include/xen/perfc_defn.h |  2 ++
 xen/include/xen/rcupdate.h   |  3 ++
 4 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index 12ae7da..871936f 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -84,8 +84,37 @@ struct rcu_data {
     int cpu;
     struct rcu_head barrier;
     long            last_rs_qlen;     /* qlen during the last resched */
+
+    /* 3) idle CPUs handling */
+    struct timer idle_timer;
+    bool idle_timer_active;
 };
 
+/*
+ * If a CPU with RCU callbacks queued goes idle, when the grace period is
+ * not finished yet, how can we make sure that the callbacks will eventually
+ * be executed? In Linux (2.6.21, the first "tickless idle" Linux kernel),
+ * the periodic timer tick would not be stopped for such CPU. Here in Xen,
+ * we (may) don't even have a periodic timer tick, so we need to use a
+ * special purpose timer.
+ *
+ * Such timer:
+ * 1) is armed only when a CPU with an RCU callback(s) queued goes idle
+ *    before the end of the current grace period (_not_ for any CPUs that
+ *    go idle!);
+ * 2) when it fires, it is only re-armed if the grace period is still
+ *    running;
+ * 3) it is stopped immediately, if the CPU wakes up from idle and
+ *    resumes 'normal' execution.
+ *
+ * About how far in the future the timer should be programmed each time,
+ * it's hard to tell (guess!!). Since this mimics Linux's periodic timer
+ * tick, take values used there as an indication. In Linux 2.6.21, tick
+ * period can be 10ms, 4ms, 3.33ms or 1ms. Let's use 10ms, to enable
+ * at least some power saving on the CPU that is going idle.
+ */
+#define RCU_IDLE_TIMER_PERIOD MILLISECS(10)
+
 static DEFINE_PER_CPU(struct rcu_data, rcu_data);
 
 static int blimit = 10;
@@ -404,7 +433,45 @@ int rcu_needs_cpu(int cpu)
 {
     struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 
-    return (!!rdp->curlist || rcu_pending(cpu));
+    return (rdp->curlist && !rdp->idle_timer_active) || rcu_pending(cpu);
+}
+
+/*
+ * Timer for making sure the CPU where a callback is queued does
+ * periodically poke rcu_pedning(), so that it will invoke the callback
+ * not too late after the end of the grace period.
+ */
+void rcu_idle_timer_start()
+{
+    struct rcu_data *rdp = &this_cpu(rcu_data);
+
+    /*
+     * Note that we don't check rcu_pending() here. In fact, we don't want
+     * the timer armed on CPUs that are in the process of quiescing while
+     * going idle, unless they really are the ones with a queued callback.
+     */
+    if (likely(!rdp->curlist))
+        return;
+
+    set_timer(&rdp->idle_timer, NOW() + RCU_IDLE_TIMER_PERIOD);
+    rdp->idle_timer_active = true;
+}
+
+void rcu_idle_timer_stop()
+{
+    struct rcu_data *rdp = &this_cpu(rcu_data);
+
+    if (likely(!rdp->idle_timer_active))
+        return;
+
+    rdp->idle_timer_active = false;
+    stop_timer(&rdp->idle_timer);
+}
+
+static void rcu_idle_timer_handler(void* data)
+{
+    /* Nothing, really... Just count the number of times we fire */
+    perfc_incr(rcu_idle_timer);
 }
 
 void rcu_check_callbacks(int cpu)
@@ -425,6 +492,8 @@ static void rcu_move_batch(struct rcu_data *this_rdp, 
struct rcu_head *list,
 static void rcu_offline_cpu(struct rcu_data *this_rdp,
                             struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 {
+    kill_timer(&rdp->idle_timer);
+
     /* If the cpu going offline owns the grace period we can block
      * indefinitely waiting for it, so flush it here.
      */
@@ -453,6 +522,7 @@ static void rcu_init_percpu_data(int cpu, struct 
rcu_ctrlblk *rcp,
     rdp->qs_pending = 0;
     rdp->cpu = cpu;
     rdp->blimit = blimit;
+    init_timer(&rdp->idle_timer, rcu_idle_timer_handler, rdp, cpu);
 }
 
 static int cpu_callback(
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index c6f4817..8827921 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1904,6 +1904,7 @@ void sched_tick_suspend(void)
     sched = per_cpu(scheduler, cpu);
     SCHED_OP(sched, tick_suspend, cpu);
     rcu_idle_enter(cpu);
+    rcu_idle_timer_start();
 }
 
 void sched_tick_resume(void)
@@ -1911,6 +1912,7 @@ void sched_tick_resume(void)
     struct scheduler *sched;
     unsigned int cpu = smp_processor_id();
 
+    rcu_idle_timer_stop();
     rcu_idle_exit(cpu);
     sched = per_cpu(scheduler, cpu);
     SCHED_OP(sched, tick_resume, cpu);
diff --git a/xen/include/xen/perfc_defn.h b/xen/include/xen/perfc_defn.h
index c135bf8..e8ff565 100644
--- a/xen/include/xen/perfc_defn.h
+++ b/xen/include/xen/perfc_defn.h
@@ -12,6 +12,8 @@ PERFCOUNTER(calls_from_multicall,       "calls from 
multicall")
 PERFCOUNTER(irqs,                   "#interrupts")
 PERFCOUNTER(ipis,                   "#IPIs")
 
+PERFCOUNTER(rcu_idle_timer,         "RCU: idle_timer")
+
 /* Generic scheduler counters (applicable to all schedulers) */
 PERFCOUNTER(sched_irq,              "sched: timer")
 PERFCOUNTER(sched_run,              "sched: runs through scheduler")
diff --git a/xen/include/xen/rcupdate.h b/xen/include/xen/rcupdate.h
index 561ac43..3402eb5 100644
--- a/xen/include/xen/rcupdate.h
+++ b/xen/include/xen/rcupdate.h
@@ -149,4 +149,7 @@ int rcu_barrier(void);
 void rcu_idle_enter(unsigned int cpu);
 void rcu_idle_exit(unsigned int cpu);
 
+void rcu_idle_timer_start(void);
+void rcu_idle_timer_stop(void);
+
 #endif /* __XEN_RCUPDATE_H */
--
generated by git-patchbot for /home/xen/git/xen.git#master

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