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@xxxxxxxxxxxxxxxxxx> --- 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(); -- 1.7.11.4 -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
