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Message-ID: <20121127130703.GW25516@redhat.com> Date: Tue, 27 Nov 2012 15:07:03 +0200 From: Gleb Natapov <gleb@...hat.com> To: Li Zhong <zhong@...ux.vnet.ibm.com> Cc: linux-next list <linux-next@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com, sasha.levin@...cle.com, avi@...hat.com, fweisbec@...il.com Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault On Tue, Nov 27, 2012 at 01:15:25PM +0800, Li Zhong wrote: > I noticed some warnings complaining about dynticks_nesting value, like > > [ 267.545032] ------------[ cut here ]------------ > [ 267.545032] WARNING: at kernel/rcutree.c:382 rcu_eqs_enter+0xab/0xc0() > [ 267.545032] Hardware name: Bochs > [ 267.545032] Modules linked in: > [ 267.545032] Pid: 0, comm: swapper/2 Not tainted 3.7.0-rc5-next-20121115 #8 > [ 267.545032] Call Trace: > [ 267.545032] [<ffffffff8104714f>] warn_slowpath_common+0x7f/0xc0 > [ 267.545032] [<ffffffff810471aa>] warn_slowpath_null+0x1a/0x20 > [ 267.545032] [<ffffffff810e607b>] rcu_eqs_enter+0xab/0xc0 > [ 267.545032] [<ffffffff810e60bb>] rcu_idle_enter+0x2b/0x70 > [ 267.545032] [<ffffffff8100d44f>] cpu_idle+0x6f/0x100 > [ 267.545032] [<ffffffff814bf055>] start_secondary+0x205/0x20c > [ 267.545032] ---[ end trace 924ae80da035028d ]--- > > After enabling rcu-dyntick tracing, I got following abnormal > dynticks_nesting values (13fffffffffffff, ff00000000000001,etc): > ... > 1 <idle>-0 [002] dN.2 18739.518567: rcu_dyntick: End 0 140000000000000 rcu_idle_exit > 2 sshd-696 [002] d..1 18739.518675: rcu_dyntick: ++= 140000000000000 140000000000001 rcu_irq_enter - apf (not present) > > 3 <idle>-0 [002] d..2 18739.518705: rcu_dyntick: Start 140000000000001 0 rcu_idle_enter > 4 <idle>-0 [002] d..2 18739.521252: rcu_dyntick: End 0 1 rcu_irq_enter - apf (page ready) > 5 <idle>-0 [002] dN.2 18739.521261: rcu_dyntick: Start 1 0 rcu_irq_exit - apf (page ready) > 6 <idle>-0 [002] dN.2 18739.521263: rcu_dyntick: End 0 140000000000000 rcu_idle_exit > > 7 sshd-696 [002] d..1 18739.521299: rcu_dyntick: --= 140000000000000 13fffffffffffff rcu_irq_exit - apf (not present) > 8 sshd-696 [002] d..1 18739.521302: rcu_dyntick: Start 13fffffffffffff 0 rcu_user_enter > 9 sshd-696 [002] d..1 18739.521330: rcu_dyntick: End 0 1 rcu_irq_enter - apf (not present) > > 10 <idle>-0 [002] d..2 18739.521346: rcu_dyntick: Start 1 0 rcu_idle_enter - old value 1, warning > 11 <idle>-0 [002] d..2 18739.530021: rcu_dyntick: ++= ff00000000000001 ff00000000000002 > 12 <idle>-0 [002] dN.2 18739.530029: rcu_dyntick: --= ff00000000000002 ff00000000000001 > ... > > I added the functions I guess which printed the tracing after each > line. > > Line #1, the idle-0 process calls rcu_idle_exit(), and finishes one > loop, to switch to sshd-696 > > Line #2, sshd-696 calls rcu_irq_enter() because of async page fault(page > not present), and puts itself to wait for page ready > > Line #3, idle-0 is switched in, and clears the dynticks_nesting to 0 > > Line #4-5, I think the rcu_irq_enter/exit() is called because the page > for sshd-696 is ready > > Line #6, idle-0 calls rcu_idle_exit(), to switch to sshd-696 > > Line #7, sshd-696 calls rcu_irq_exit() in the apf (page not present) > code path, decreasing dynticks_nesting to 13fffffffffffff. > > Line #8-9, sshd-696 calls rcu_user_enter() to start user eqs, and gets > async page fault again. It puts itself sleep again, with > dynticks_nesting value as 1. > > Line #10, idle-0 switches in, as the dynticks_nesting value is 1, so > warning is reported in rcu_idle_enter(), then the value is decreased to > ff00000000000001. (In the tracing log, the new value is 0, that's > because rcu hard-code the value to be 0. I will send another patch for > this.) > > This patch below tries to replace the rcu_irq_enter/exit() with > rcu_idle_exit/enter(), if it is in rcu idle, and it is idle process; > otherwise, rcu_user_exit() is called to exit user eqs if necessary. > > Signed-off-by: Li Zhong <zhong@...ux.vnet.ibm.com> > --- > arch/x86/kernel/kvm.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c > index 4180a87..f65648d 100644 > --- a/arch/x86/kernel/kvm.c > +++ b/arch/x86/kernel/kvm.c > @@ -247,10 +247,17 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code) > break; > case KVM_PV_REASON_PAGE_NOT_PRESENT: > /* page is swapped out by the host. */ > - rcu_irq_enter(); > + if (is_idle_task(current) && rcu_is_cpu_idle()) > + rcu_idle_exit(); > + else > + rcu_user_exit(); > + > exit_idle(); > kvm_async_pf_task_wait((u32)read_cr2()); > - rcu_irq_exit(); > + > + if (is_idle_task(current) && rcu_is_cpu_idle()) > + rcu_idle_enter(); > + > break; > case KVM_PV_REASON_PAGE_READY: > rcu_irq_enter(); Those rcu_irq_enter()/rcu_irq_exit() were introduced by commit c5e015d4949aa665 "KVM guest: exit idleness when handling KVM_PV_REASON_PAGE_NOT_PRESENT", but now I am starting to question this commit. KVM_PV_REASON_PAGE_NOT_PRESENT should not kick cpu out of idleness. kvm_async_pf_task_wait() checks that cpu is idle and calls halt if it is. After that commit schedule() may be called between rcu_irq_enter()/rcu_irq_exit() which is probably illegal. Paul? -- Gleb. -- 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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