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Message-ID: <20110926084800.GB18553@somewhere> Date: Mon, 26 Sep 2011 10:48:04 +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 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. Oh right. That seem to exclude any tracing in idle. > > So if I understand correctly we would check if we are in an rcu read side > > critical section when we call rcu_needs_cpu(). If so then we keep > > the tick alive. Afterward when we exit the rcu read side critical section > > (rcu_read_unlock/local_bh_enable), we notice that specific state and > > we try to accelerate the rcu callbacks processing from there to switch > > to dynticks idle mode, right? > > > > So that requires some specific counter in rcu_read_lock() for the > > !CONFIG_PREEMPT case so that we know if we are interrupting an > > rcu read side critical section from rcu_needs_cpu(). For the > > bh case we probably can just check in_softirq(). > > > > Also if we know we are interrupting a read side section, why not just > > keep the tick alive and retry the next tick? Interrupting such > > section looks rare enough that it wouldn't have much impact > > and that avoids specific hooks in rcu_read_unlock() and local_bh_enable(). > > Good point. Perhaps only bother with this if returning to idle, then? 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