Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage basedon semi-formal proof"
From: Yinghai Lu
Date: Tue May 24 2011 - 17:24:16 EST
On 05/23/2011 06:35 PM, Paul E. McKenney wrote:
> On Mon, May 23, 2011 at 06:26:23PM -0700, Yinghai Lu wrote:
>> On 05/23/2011 06:18 PM, Paul E. McKenney wrote:
>>
>>> OK, so it looks like I need to get this out of the way in order to track
>>> down the delays. Or does reverting PeterZ's patch get you a stable
>>> system, but with the longish delays in memory_dev_init()? If the latter,
>>> it might be more productive to handle the two problems separately.
>>>
>>> For whatever it is worth, I do see about 5% increase in grace-period
>>> duration when switching to kthreads. This is acceptable -- your
>>> 30x increase clearly is completely unacceptable and must be fixed.
>>> Other than that, the main thing that affects grace period duration is
>>> the setting of CONFIG_HZ -- the smaller the HZ value, the longer the
>>> grace-period duration.
>>
>> for my 1024g system when memory hotadd is enabled in kernel config:
>> 1. current linus tree + tip tree: memory_dev_init will take about 100s.
>> 2. current linus tree + tip tree + your tree - Peterz patch:
>> a. on fedora 14 gcc: will cost about 4s: like old times
>> b. on opensuse 11.3 gcc: will cost about 10s.
>
> So some patch in my tree that is not yet in tip makes things better?
>
> If so, could you please see which one? Maybe that would give me a hint
> that could make things better on opensuse 11.3 as well.
today's tip:
[ 31.795597] cpu_dev_init done
[ 40.930202] memory_dev_init done
after
commit e219b351fc90c0f5304e16efbc603b3b78843ea1
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon May 16 02:44:06 2011 -0700
rcu: Remove old memory barriers from rcu_process_callbacks()
Second step of partitioning of commit e59fb3120b.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3731141..011bf6f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1460,25 +1460,11 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
*/
static void rcu_process_callbacks(void)
{
- /*
- * Memory references from any prior RCU read-side critical sections
- * executed by the interrupted code must be seen before any RCU
- * grace-period manipulations below.
- */
- smp_mb(); /* See above block comment. */
-
__rcu_process_callbacks(&rcu_sched_state,
&__get_cpu_var(rcu_sched_data));
__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
rcu_preempt_process_callbacks();
- /*
- * Memory references from any later RCU read-side critical sections
- * executed by the interrupted code must be seen after any RCU
- * grace-period manipulations above.
- */
- smp_mb(); /* See above block comment. */
-
/* If we are last CPU on way to dyntick-idle mode, accelerate it. */
rcu_needs_cpu_flush();
}
cause
[ 32.235103] cpu_dev_init done
[ 74.897943] memory_dev_init done
then add
commit d0d642680d4cf5cc2ccf542b74a3c8b7e197306b
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon May 16 02:52:04 2011 -0700
rcu: Don't do reschedule unless in irq
Condition the set_need_resched() in rcu_irq_exit() on in_irq(). This
should be a no-op, because rcu_irq_exit() should only be called from irq.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 011bf6f..195b3a3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -421,8 +421,9 @@ void rcu_irq_exit(void)
WARN_ON_ONCE(rdtp->dynticks & 0x1);
/* If the interrupt queued a callback, get out of dyntick mode. */
- if (__this_cpu_read(rcu_sched_data.nxtlist) ||
- __this_cpu_read(rcu_bh_data.nxtlist))
+ if (in_irq() &&
+ (__this_cpu_read(rcu_sched_data.nxtlist) ||
+ __this_cpu_read(rcu_bh_data.nxtlist)))
set_need_resched();
}
got:
[ 34.384490] cpu_dev_init done
[ 86.656322] memory_dev_init done
after
commit fcfc28801f5b3b9c70616fc57e3a2c6f52014e14
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon May 16 14:27:31 2011 -0700
rcu: Make rcu_enter_nohz() pay attention to nesting
The old version of rcu_enter_nohz() forced RCU into nohz mode even if
the nesting count was non-zero. This change causes rcu_enter_nohz()
to hold off for non-zero nesting counts.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 195b3a3..99c6038 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -324,8 +324,8 @@ void rcu_enter_nohz(void)
smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
local_irq_save(flags);
rdtp = &__get_cpu_var(rcu_dynticks);
- rdtp->dynticks++;
- rdtp->dynticks_nesting--;
+ if (--rdtp->dynticks_nesting == 0)
+ rdtp->dynticks++;
WARN_ON_ONCE(rdtp->dynticks & 0x1);
local_irq_restore(flags);
}
got:
[ 32.414049] cpu_dev_init done
[ 38.237979] memory_dev_init done
after:
commit bcd6e68330f893a81b3519ab3c5fc2bebbc9988c
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Tue Sep 7 10:38:22 2010 -0700
rcu: Decrease memory-barrier usage based on semi-formal proof
...
got:
[ 32.447936] cpu_dev_init done
[ 111.027066] memory_dev_init done
after
commit fbb753fb9dd62318d27fa070c686423ced139817
Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Date: Wed May 11 05:33:33 2011 -0700
atomic: Add atomic_or()
An atomic_or() function is needed by TREE_RCU to avoid deadlock, so
add a generic version.
Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 96c038e..ee456c7 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -34,4 +34,17 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
}
#endif
+#ifndef CONFIG_ARCH_HAS_ATOMIC_OR
+static inline void atomic_or(int i, atomic_t *v)
+{
+ int old;
+ int new;
+
+ do {
+ old = atomic_read(v);
+ new = old | i;
+ } while (atomic_cmpxchg(v, old, new) != old);
+}
+#endif /* #ifndef CONFIG_ARCH_HAS_ATOMIC_OR */
+
#endif /* _LINUX_ATOMIC_H */
got:
[ 32.803704] cpu_dev_init done
[ 99.171292] memory_dev_init done
--
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