Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usagebased on semi-formal proof"

From: Paul E. McKenney
Date: Wed May 25 2011 - 00:47:10 EST

On Tue, May 24, 2011 at 05:13:06PM -0700, Yinghai Lu wrote:
> 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

Use of synchronize_rcu() for unregistering would make sense, but
I don't understand why it is needed when registering.

Thanx, Paul

> /*
> * 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@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
> >
> > 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@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
> >
> > 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@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
> >
> > 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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