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Date: Tue, 24 May 2011 17:13:06 -0700
From: Yinghai Lu <yinghai@...nel.org>
To: paulmck@...ux.vnet.ibm.com
CC: linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based
on semi-formal proof"
On 05/24/2011 05:05 PM, Paul E. McKenney wrote:
> On Tue, May 24, 2011 at 02:23:45PM -0700, Yinghai Lu wrote:
>> 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
>
> One other question... What is memory_dev_init() doing to wait for so
> many RCU grace periods? (Yes, I do need to fix the slowdowns in any
> case, but I am curious.)
looks like it register some in sysfs
/*
* Initialize the sysfs support for memory devices...
*/
int __init memory_dev_init(void)
{
unsigned int i;
int ret;
int err;
unsigned long block_sz;
memory_sysdev_class.kset.uevent_ops = &memory_uevent_ops;
ret = sysdev_class_register(&memory_sysdev_class);
if (ret)
goto out;
block_sz = get_memory_block_size();
sections_per_block = block_sz / MIN_MEMORY_BLOCK_SIZE;
/*
* Create entries for memory sections that were found
* during boot and have been initialized
*/
for (i = 0; i < NR_MEM_SECTIONS; i++) {
if (!present_section_nr(i))
continue;
err = add_memory_section(0, __nr_to_section(i), MEM_ONLINE,
BOOT);
if (!ret)
ret = err;
}
err = memory_probe_init();
if (!ret)
ret = err;
err = memory_fail_init();
if (!ret)
ret = err;
err = block_size_init();
if (!ret)
ret = err;
out:
if (ret)
printk(KERN_ERR "%s() failed: %d\n", __func__, ret);
return ret;
}
>
>> after
>>
>> commit e219b351fc90c0f5304e16efbc603b3b78843ea1
>> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> 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@...ux.vnet.ibm.com>
>>
>> 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@...ux.vnet.ibm.com>
>> 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@...ux.vnet.ibm.com>
>>
>> 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@...ux.vnet.ibm.com>
>> 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@...ux.vnet.ibm.com>
>>
>> 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
>
> So this is best for you -- where we have done all but the last commit
> of restoring "Decrease memory-barrier usage based on semi-formal proof".
> It makes sense that this one would help, as it is eliminating delays
> due to misnesting. These delays are not hangs, as force_quiescent_state()
> will eventually force the right thing to happen, but getting rid of these
> delays should indeed speed things up.
>
>> after:
>> commit bcd6e68330f893a81b3519ab3c5fc2bebbc9988c
>> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> 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
>
> So there is something nasty in this patch.
>
> Not seeing it immediately, but it does give me some focus for both
> code inspection and possible diagnostic patches.
>
>> after
>>
>> commit fbb753fb9dd62318d27fa070c686423ced139817
>> Author: Paul E. McKenney <paul.mckenney@...aro.org>
>> 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@...aro.org>
>> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>
>> 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
>
> So the difference between these two is noise, I hope. Adding a static
> inline function that is not used should not have an effect on performance.
> Still, the difference between 6 seconds and 60 seconds rises far above
> this noise level, so the big differences are likely quite real.
could be softirq to kthread change...
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