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Message-ID: <20110927115500.GI18553@somewhere> Date: Tue, 27 Sep 2011 13:55:03 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: "Kirill A. Shutemov" <kirill@...temov.name>, linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Lai Jiangshan <laijs@...fujitsu.com> Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833 On Mon, Sep 26, 2011 at 03:30:10PM -0700, Paul E. McKenney wrote: > On Mon, Sep 26, 2011 at 10:49:53AM +0200, Frederic Weisbecker wrote: > > On Sun, Sep 25, 2011 at 06:25:01PM -0700, Paul E. McKenney wrote: > > > On Mon, Sep 26, 2011 at 03:04:21AM +0200, Frederic Weisbecker wrote: > > > > On Sun, Sep 25, 2011 at 09:48:04AM -0700, Paul E. McKenney wrote: > > > > > On Sun, Sep 25, 2011 at 03:06:25PM +0200, Frederic Weisbecker wrote: > > > > > > On Sun, Sep 25, 2011 at 02:26:37PM +0300, Kirill A. Shutemov wrote: > > > > > > > On Sat, Sep 24, 2011 at 10:08:26PM -0700, Paul E. McKenney wrote: > > > > > > > > On Sun, Sep 25, 2011 at 03:24:09AM +0300, Kirill A. Shutemov wrote: > > > > > > > > > [ 29.974288] ------------[ cut here ]------------ > > > > > > > > > [ 29.974308] WARNING: at /home/kas/git/public/linux-next/kernel/rcutree.c:1833 rcu_needs_cpu+0xff > > > > > > > > > [ 29.974316] Hardware name: HP EliteBook 8440p > > > > > > > > > [ 29.974321] Modules linked in: ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iple_mangle xt_tcpudp iptable_filter ip_tables x_tables bridge stp llc rfcomm bnep acpi_cpufreq mperfckd fscache auth_rpcgss nfs_acl sunrpc ext2 loop kvm_intel kvm snd_hda_codec_hdmi snd_hda_codec_idtideodev media v4l2_compat_ioctl32 snd_seq bluetooth drm_kms_helper snd_timer tpm_infineon snd_seq_drt tpm_tis hp_accel intel_ips soundcore lis3lv02d tpm rfkill i2c_algo_bit snd_page_alloc i2c_core c16 sha256_generic aesni_intel cryptd aes_x86_64 aes_generic cbc dm_crypt dm_mod sg sr_mod sd_mod cd thermal_sys [last unloaded: scsi_wait_scan] > > > > > > > > > [ 29.974517] Pid: 0, comm: kworker/0:1 Not tainted 3.1.0-rc7-next-20110923 #2 > > > > > > > > > [ 29.974521] Call Trace: > > > > > > > > > [ 29.974525] <IRQ> [<ffffffff8104d72a>] warn_slowpath_common+0x7a/0xb0 > > > > > > > > > [ 29.974540] [<ffffffff8104d775>] warn_slowpath_null+0x15/0x20 > > > > > > > > > [ 29.974546] [<ffffffff810bffdf>] rcu_needs_cpu+0xff/0x110 > > > > > > > > > [ 29.974555] [<ffffffff8108396f>] tick_nohz_stop_sched_tick+0x13f/0x3d0 > > > > > > > > > [ 29.974563] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70 > > > > > > > > > [ 29.974571] [<ffffffff81055622>] irq_exit+0xa2/0xd0 > > > > > > > > > [ 29.974578] [<ffffffff8101ee75>] smp_apic_timer_interrupt+0x85/0x1c0 > > > > > > > > > [ 29.974585] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70 > > > > > > > > > [ 29.974592] [<ffffffff81436e1e>] apic_timer_interrupt+0x6e/0x80 > > > > > > > > > [ 29.974596] <EOI> [<ffffffff81297abd>] ? acpi_hw_read+0x4a/0x51 > > > > > > > > > [ 29.974609] [<ffffffff81087a07>] ? lock_acquire+0xa7/0x160 > > > > > > > > > [ 29.974615] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70 > > > > > > > > > [ 29.974622] [<ffffffff81432a16>] __atomic_notifier_call_chain+0x56/0xb0 > > > > > > > > > [ 29.974631] [<ffffffff814329c0>] ? notifier_call_chain+0x70/0x70 > > > > > > > > > [ 29.974642] [<ffffffff8130ebb6>] ? cpuidle_idle_call+0x106/0x350 > > > > > > > > > [ 29.974651] [<ffffffff81432a81>] atomic_notifier_call_chain+0x11/0x20 > > > > > > > > > [ 29.974661] [<ffffffff81001233>] cpu_idle+0xe3/0x120 > > > > > > > > > [ 29.974672] [<ffffffff8141e34b>] start_secondary+0x1fd/0x204 > > > > > > > > > [ 29.974681] ---[ end trace 6c1d44095a3bb7c5 ]--- > > > > > > > > > > > > > > > > Do the following help? > > > > > > > > > > > > > > > > https://lkml.org/lkml/2011/9/17/47 > > > > > > > > https://lkml.org/lkml/2011/9/17/45 > > > > > > > > https://lkml.org/lkml/2011/9/17/43 > > > > > > > > > > > > > > Yes. Thanks. > > > > > > > > > > > > I believe that doesn't really fix the issue. But the warning is not > > > > > > easy to trigger. You simply haven't hit it by chance after applying > > > > > > the patches. > > > > > > > > > > > > This happens when the idle notifier callchain is called in idle > > > > > > and is interrupted in the middle. So we have called rcu_read_lock() > > > > > > but haven't yet released with rcu_read_unlock(), and in the end > > > > > > of the interrupt we call tick_nohz_stop_sched_tick() -> rcu_needs_cpu() > > > > > > which is illegal while in an rcu read side critical section. > > > > > > > > > > > > No idea how to solve that. Any use of RCU after the tick gets stopped > > > > > > is concerned here. If it is really required that rcu_needs_cpu() can't > > > > > > be called in an rcu read side critical sectionn then it's not going > > > > > > to be easy to fix. > > > > > > > > > > > > But I don't really understand that requirement. rcu_needs_cpu() simply > > > > > > checks if we don't have callbacks to handle. So I don't understand how > > > > > > read side is concerned. It's rather the write side. > > > > > > The rule I can imagine instead is: don't call __call_rcu() once the tick is > > > > > > stopped. > > > > > > > > > > > > But I'm certainly missing something. > > > > > > > > > > > > Paul? > > > > > > > > > > This is required for RCU_FAST_NO_HZ, which checks to see whether the > > > > > current CPU can accelerate the current grace period so as to enter > > > > > dyntick-idle mode sooner than it would otherwise. This takes effect > > > > > in the situation where rcu_needs_cpu() sees that there are callbacks. > > > > > It then notes a quiescent state (which is illegal in an RCU read-side > > > > > critical section), calls force_quiescent_state(), and so on. For this > > > > > to work, the current CPU must be in an RCU read-side critical section. > > > > > > > > You mean it must *not* be in an RCU read-side critical section (ie: in a > > > > quiescent state)? > > > > > > Yes, you are right, it must -not- be in an RCU read-side critical section. > > > > > > > That assumption at least fails anytime in idle for the RCU > > > > sched flavour given that preemption is disabled in the idle loop. > > > > > > Except that the idle loop is a quiescent state. > > > > > > > > If this cannot be made to work, another option is to call a new RCU > > > > > function in the case where rcu_needs_cpu() returned false, but after > > > > > the RCU read-side critical section has exited. > > > > > > > > You mean when rcu_needs_cpu() returns true (when we have callbacks > > > > enqueued)? > > > > > > Yes. I definitely am having problems with polarity this weekend. :-/ > > > > > > > > This new RCU function > > > > > could then attempt to rearrange RCU so as to allow the CPU to enter > > > > > dyntick-idle mode more quickly. It is more important for this to > > > > > happen when the CPU is going idle than when it is executing a user > > > > > process. > > > > > > > > > > So, is this doable? > > > > > > > > At least not when we have RCU sched callbacks enqueued, given preemption > > > > is disabled. But that sounds plausible in order to accelerate the switch > > > > to dyntick-idle mode when we only have rcu and/or rcu bh callbacks. > > > > > > Again, the idle loop is a quiescent state for RCU-sched. > > > > We need to add an idle_cpu() check in rcu_read_lock_sched_held() > > and rcu_read_lock_bh_held(). > > OK, but just to make sure... For this to work, the idle notifiers would > need to be invoked from outside of the idle task. If the idle notifiers > are instead invoked from idle-task context, then RCU needs to get more > precise about what it thinks of as "idle", probably via a per-CPU variable > that gets set and cleared within each idle loop. But the idle notifiers run under RCU RCU, not RCU sched. Is the fact we are in idle a quiescent state also for that flavor? Hmm, may be in the !PREEMPT case? :-s > > /me reminisces about the time when the definition of "idle" seemed so > simple... ;-) ;-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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