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Subject[RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault
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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@linux.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();
--
1.7.11.4


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