From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933313Ab1EYANZ (ORCPT ); Tue, 24 May 2011 20:13:25 -0400 Received: from rcsinet10.oracle.com ([148.87.113.121]:18918 "EHLO rcsinet10.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752365Ab1EYANX (ORCPT ); Tue, 24 May 2011 20:13:23 -0400 Message-ID: <4DDC4992.2020505@kernel.org> Date: Tue, 24 May 2011 17:13:06 -0700 From: Yinghai Lu User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110414 SUSE/3.1.10 Thunderbird/3.1.10 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com, tglx@linutronix.de, mingo@elte.hu Subject: Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof" References: <20110521140845.GA12157@linux.vnet.ibm.com> <4DDAC01E.7050602@kernel.org> <20110523212530.GF7428@linux.vnet.ibm.com> <4DDAD934.9010603@kernel.org> <4DDAE5FA.2030303@kernel.org> <4DDAE6A5.6060701@kernel.org> <20110524011824.GL7428@linux.vnet.ibm.com> <4DDB093F.2060601@kernel.org> <20110524013523.GO7428@linux.vnet.ibm.com> <4DDC21E1.1070502@kernel.org> <20110525000530.GK2266@linux.vnet.ibm.com> In-Reply-To: <20110525000530.GK2266@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Source-IP: acsinet22.oracle.com [141.146.126.238] X-Auth-Type: Internal IP X-CT-RefId: str=0001.0A090203.4DDC4996.009D,ss=1,fgs=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> 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 >> >> 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 >> 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 >> >> 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 >> 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 >> >> 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 >> 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 >> 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 >> Signed-off-by: Paul E. McKenney >> >> 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...