OpenCloudOS-Kernel/kernel/rcu
Alexander Gordeev ca9558a33f rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU
Let's start assuming that something in the idle loop posts a callback,
and scheduling-clock interrupt occurs:

1. The system is idle and stays that way, no runnable tasks.

2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called
   as result, which in turn calls rcu_is_cpu_rrupt_from_idle().

3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from
   idle, which results in rcu_sched_qs() call, which does a
   raise_softirq(RCU_SOFTIRQ).

4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls
   rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again,
   which does another raise_softirq(RCU_SOFTIRQ).

5. The softirq happens shortly and invokes rcu_process_callbacks(),
   which invokes __rcu_process_callbacks().

6. So now callbacks can be invoked. At least they can be if
   ->donetail has been updated. Which it will have been because
   rcu_sched_qs() invokes rcu_qsctr_help().

In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ)
get called twice in steps 3 and 4. This redundancy could be eliminated
by removing rcu_is_cpu_rrupt_from_idle() function.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-12-30 17:40:18 -08:00
..
Makefile rcu: Remove redundant TREE_PREEMPT_RCU config option 2014-10-29 10:20:05 -07:00
rcu.h rcutorture: Add early boot self tests 2014-11-03 19:26:37 -08:00
rcutorture.c rcutorture: Fix rcu_torture_cbflood() memory leak 2014-11-03 19:26:41 -08:00
srcu.c rcu: Eliminate read-modify-write ACCESS_ONCE() calls 2014-07-09 09:14:49 -07:00
tiny.c rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU 2014-12-30 17:40:18 -08:00
tiny_plugin.h rcu: Protect uses of ->jiffies_stall with ACCESS_ONCE() 2014-04-29 08:44:41 -07:00
tree.c rcu: Make rcu_nmi_enter() handle nesting 2014-12-30 17:40:16 -08:00
tree.h Merge branches 'torture.2014.11.03a', 'cpu.2014.11.03a', 'doc.2014.11.13a', 'fixes.2014.11.13a', 'signal.2014.10.29a' and 'rt.2014.10.29a' into HEAD 2014-11-13 10:39:04 -08:00
tree_plugin.h Merge branches 'torture.2014.11.03a', 'cpu.2014.11.03a', 'doc.2014.11.13a', 'fixes.2014.11.13a', 'signal.2014.10.29a' and 'rt.2014.10.29a' into HEAD 2014-11-13 10:39:04 -08:00
tree_trace.c rcu: Stop tracking FSF's postal address 2014-02-17 15:01:37 -08:00
update.c Merge branches 'torture.2014.11.03a', 'cpu.2014.11.03a', 'doc.2014.11.13a', 'fixes.2014.11.13a', 'signal.2014.10.29a' and 'rt.2014.10.29a' into HEAD 2014-11-13 10:39:04 -08:00