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Re: [Xen-devel] [PATCH v5 1/4] xen/rcu: don't use stop_machine_run() for rcu_barrier()



On 13.03.20 12:02, Julien Grall wrote:


On 12/03/2020 08:28, Juergen Gross wrote:
Today rcu_barrier() is calling stop_machine_run() to synchronize all
physical cpus in order to ensure all pending rcu calls have finished
when returning.

As stop_machine_run() is using tasklets this requires scheduling of
idle vcpus on all cpus imposing the need to call rcu_barrier() on idle
cpus only in case of core scheduling being active, as otherwise a
scheduling deadlock would occur.

There is no need at all to do the syncing of the cpus in tasklets, as
rcu activity is started in __do_softirq() called whenever softirq
activity is allowed. So rcu_barrier() can easily be modified to use
softirq for synchronization of the cpus no longer requiring any
scheduling activity.

As there already is a rcu softirq reuse that for the synchronization.

Remove the barrier element from struct rcu_data as it isn't used.

Finally switch rcu_barrier() to return void as it now can never fail.

Partially-based-on-patch-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V2:
- add recursion detection

V3:
- fix races (Igor Druzhinin)

V5:
- rename done_count to pending_count (Jan Beulich)
- fix race (Jan Beulich)
---
  xen/common/rcupdate.c      | 92 ++++++++++++++++++++++++++++++++--------------
  xen/include/xen/rcupdate.h |  2 +-
  2 files changed, 66 insertions(+), 28 deletions(-)

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index 03d84764d2..c5ef6acb1e 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -83,7 +83,6 @@ struct rcu_data {
      struct rcu_head **donetail;
      long            blimit;           /* Upper limit on a processed batch */
      int cpu;
-    struct rcu_head barrier;
      long            last_rs_qlen;     /* qlen during the last resched */
      /* 3) idle CPUs handling */
@@ -91,6 +90,7 @@ struct rcu_data {
      bool idle_timer_active;
      bool            process_callbacks;
+    bool            barrier_active;
  };
  /*
@@ -143,51 +143,82 @@ static int qhimark = 10000;
  static int qlowmark = 100;
  static int rsinterval = 1000;
-struct rcu_barrier_data {
-    struct rcu_head head;
-    atomic_t *cpu_count;
-};
+/*
+ * rcu_barrier() handling:
+ * cpu_count holds the number of cpu required to finish barrier handling.

NIT: the number of cpus (I think)

+ * pending_count is initialized to nr_cpus + 1.
+ * Cpus are synchronized via softirq mechanism. rcu_barrier() is regarded to + * be active if pending_count is not zero. In case rcu_barrier() is called on + * multiple cpus it is enough to check for pending_count being not zero on entry + * and to call process_pending_softirqs() in a loop until pending_count drops to
+ * zero, before starting the new rcu_barrier() processing.
+ * In order to avoid hangs when rcu_barrier() is called multiple times on the
+ * same cpu in fast sequence and a slave cpu couldn't drop out of the
+ * barrier handling fast enough a second counter pending_count is needed.

As an aside question, don't we miss a memory barrier in rcu_barrier_callback or rcu_barrier_action()? This barrier would ensure that the RCU changes have been seen before we tell the "master" CPU we are done.

Sounds like a sensible idea.


+ * The rcu_barrier() invoking cpu will wait until pending_count reaches 1 + * (meaning that all cpus have finished processing the barrier) and then will
+ * reset pending_count to 0 to enable entering rcu_barrier() again.
+ */
+static atomic_t cpu_count = ATOMIC_INIT(0);
+static atomic_t pending_count = ATOMIC_INIT(0);
  static void rcu_barrier_callback(struct rcu_head *head)
  {
-    struct rcu_barrier_data *data = container_of(
-        head, struct rcu_barrier_data, head);
-    atomic_inc(data->cpu_count);
+    atomic_dec(&cpu_count);
  }
-static int rcu_barrier_action(void *_cpu_count)
+static void rcu_barrier_action(void)
  {
-    struct rcu_barrier_data data = { .cpu_count = _cpu_count };
-
-    ASSERT(!local_irq_is_enabled());
-    local_irq_enable();
+    struct rcu_head head;
      /*
       * When callback is executed, all previously-queued RCU work on this CPU -     * is completed. When all CPUs have executed their callback, data.cpu_count
-     * will have been incremented to include every online CPU.
+     * is completed. When all CPUs have executed their callback, cpu_count
+     * will have been decremented to 0.
       */
-    call_rcu(&data.head, rcu_barrier_callback);
+    call_rcu(&head, rcu_barrier_callback);
-    while ( atomic_read(data.cpu_count) != num_online_cpus() )
+    while ( atomic_read(&cpu_count) )
      {
          process_pending_softirqs();
          cpu_relax();
      }
-    local_irq_disable();
-
-    return 0;
+    atomic_dec(&pending_count);
  }
-/*
- * As rcu_barrier() is using stop_machine_run() it is allowed to be used in
- * idle context only (see comment for stop_machine_run()).
- */
-int rcu_barrier(void)
+void rcu_barrier(void)
  {
-    atomic_t cpu_count = ATOMIC_INIT(0);
-    return stop_machine_run(rcu_barrier_action, &cpu_count, NR_CPUS);
+    unsigned int n_cpus;
+

It would be good to spell out this code has to be called with interrupt enabled and not in an interrupt context.

I'll add an ASSERT().


+    for ( ;; )
+    {
+        if ( !atomic_read(&pending_count) && get_cpu_maps() )
+        {
+            n_cpus = num_online_cpus();
+
+            if ( atomic_cmpxchg(&pending_count, 0, n_cpus + 1) == 0 )
+                break;
+
+            put_cpu_maps();
+        }
+
+        process_pending_softirqs();
+        cpu_relax();
+    }
+
+    atomic_set(&cpu_count, n_cpus);
+    cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
+
+    put_cpu_maps();

If you put the CPU maps, wouldn't it be possible to have a CPU turned off? If not, can you add a comment in the code why this is safe?

Yes, you are right. This might be possible, even if rather
unlikely as a cpu being removed has to be in idle already, so
the pending softirq should be picked up rather fast.


Juergen

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