Re: [Xen-devel] [PATCH v2 0/4] xen/rcu: let rcu work better with core scheduling

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 22/02/2020 06:05, Jürgen Groß wrote:
> On 22.02.20 03:29, Igor Druzhinin wrote:
>> On 18/02/2020 12:21, Juergen Gross wrote:
>>> Today the RCU handling in Xen is affecting scheduling in several ways.
>>> It is raising sched softirqs without any real need and it requires
>>> tasklets for rcu_barrier(), which interacts badly with core scheduling.
>>>
>>> This small series repairs those issues.
>>>
>>> Additionally some ASSERT()s are added for verification of sane rcu
>>> handling. In order to avoid those triggering right away the obvious
>>> violations are fixed.
>>
>> I've done more testing of this with [1] and, unfortunately, it quite easily
>> deadlocks while without this series it doesn't.
>>
>> Steps to repro:
>> - apply [1]
>> - take a host with considerable CPU count (~64)
>> - run a loop: xen-hptool smt-disable; xen-hptool smt-enable
>>
>> [1] 
>> https://lists.xenproject.org/archives/html/xen-devel/2020-02/msg01383.html
> 
> Yeah, the reason for that is that rcu_barrier() is a nop in this
> situation without my patch, as the then called stop_machine_run() in
> rcu_barrier() will just return -EBUSY.

Are you sure that's ther reason? I always have the following stack on CPU0:

(XEN) [  120.891143] *** Dumping CPU0 host state: ***
(XEN) [  120.895909] ----[ Xen-4.13.0  x86_64  debug=y   Not tainted ]----
(XEN) [  120.902487] CPU:    0
(XEN) [  120.905269] RIP:    e008:[<ffff82d0802aa750>] 
smp_send_call_function_mask+0x40/0x43
(XEN) [  120.913415] RFLAGS: 0000000000000286   CONTEXT: hypervisor
(XEN) [  120.919389] rax: 0000000000000000   rbx: ffff82d0805ddb78   rcx: 
0000000000000001
(XEN) [  120.927362] rdx: ffff82d0805cdb00   rsi: ffff82d0805c7cd8   rdi: 
0000000000000007
(XEN) [  120.935341] rbp: ffff8300920bfbc0   rsp: ffff8300920bfbb8   r8:  
000000000000003b
(XEN) [  120.943310] r9:  0444444444444432   r10: 3333333333333333   r11: 
0000000000000001
(XEN) [  120.951282] r12: ffff82d0805ddb78   r13: 0000000000000001   r14: 
ffff8300920bfc18
(XEN) [  120.959251] r15: ffff82d0802af646   cr0: 000000008005003b   cr4: 
00000000003506e0
(XEN) [  120.967223] cr3: 00000000920b0000   cr2: ffff88820dffe7f8
(XEN) [  120.973125] fsb: 0000000000000000   gsb: ffff88821e3c0000   gss: 
0000000000000000
(XEN) [  120.981094] ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e010   cs: 
e008
(XEN) [  120.988548] Xen code around <ffff82d0802aa750> 
(smp_send_call_function_mask+0x40/0x43):
(XEN) [  120.997037]  85 f9 ff fb 48 83 c4 08 <5b> 5d c3 9c 58 f6 c4 02 74 02 
0f 0b 55 48 89 e5
(XEN) [  121.005442] Xen stack trace from rsp=ffff8300920bfbb8:
(XEN) [  121.011080]    ffff8300920bfc18 ffff8300920bfc00 ffff82d080242c84 
ffff82d080389845
(XEN) [  121.019145]    ffff8300920bfc18 ffff82d0802af178 0000000000000000 
0000001c1d27aff8
(XEN) [  121.027200]    0000000000000000 ffff8300920bfc80 ffff82d0802af1fa 
ffff82d080289adf
(XEN) [  121.035255]    fffffffffffffd55 0000000000000000 0000000000000000 
0000000000000000
(XEN) [  121.043320]    0000000000000000 0000000000000000 0000000000000000 
0000000000000000
(XEN) [  121.051375]    000000000000003b 0000001c25e54bf1 0000000000000000 
ffff8300920bfc80
(XEN) [  121.059443]    ffff82d0805c7300 ffff8300920bfcb0 ffff82d080245f4d 
ffff82d0802af4a2
(XEN) [  121.067498]    ffff82d0805c7300 ffff83042bb24f60 ffff82d08060f400 
ffff8300920bfd00
(XEN) [  121.075553]    ffff82d080246781 ffff82d0805cdb00 ffff8300920bfd80 
ffff82d0805c7040
(XEN) [  121.083621]    ffff82d0805cdb00 ffff82d0805cdb00 fffffffffffffff9 
ffff8300920bffff
(XEN) [  121.091674]    0000000000000000 ffff8300920bfd30 ffff82d0802425a5 
ffff82d0805c7040
(XEN) [  121.099739]    ffff82d0805cdb00 fffffffffffffff9 ffff8300920bffff 
ffff8300920bfd40
(XEN) [  121.107797]    ffff82d0802425e5 ffff8300920bfd80 ffff82d08022bc0f 
0000000000000000
(XEN) [  121.115852]    ffff82d08022b600 ffff82d0804b3888 ffff82d0805cdb00 
ffff82d0805cdb00
(XEN) [  121.123917]    fffffffffffffff9 ffff8300920bfdb0 ffff82d0802425a5 
0000000000000003
(XEN) [  121.131975]    0000000000000001 00000000ffffffef ffff8300920bffff 
ffff8300920bfdc0
(XEN) [  121.140037]    ffff82d0802425e5 ffff8300920bfdd0 ffff82d08022b91b 
ffff8300920bfdf0
(XEN) [  121.148093]    ffff82d0802addb1 ffff83042b3b0000 0000000000000003 
ffff8300920bfe30
(XEN) [  121.156150]    ffff82d0802ae086 ffff8300920bfe10 ffff83042b7e81e0 
ffff83042b3b0000
(XEN) [  121.164216]    0000000000000000 0000000000000000 0000000000000000 
ffff8300920bfe50
(XEN) [  121.172271] Xen call trace:
(XEN) [  121.175573]    [<ffff82d0802aa750>] R 
smp_send_call_function_mask+0x40/0x43
(XEN) [  121.183024]    [<ffff82d080242c84>] F on_selected_cpus+0xa4/0xde
(XEN) [  121.189520]    [<ffff82d0802af1fa>] F 
arch/x86/time.c#time_calibration+0x82/0x89
(XEN) [  121.197403]    [<ffff82d080245f4d>] F 
common/timer.c#execute_timer+0x49/0x64
(XEN) [  121.204951]    [<ffff82d080246781>] F 
common/timer.c#timer_softirq_action+0x116/0x24e
(XEN) [  121.213271]    [<ffff82d0802425a5>] F 
common/softirq.c#__do_softirq+0x85/0x90
(XEN) [  121.220890]    [<ffff82d0802425e5>] F 
process_pending_softirqs+0x35/0x37
(XEN) [  121.228086]    [<ffff82d08022bc0f>] F 
common/rcupdate.c#rcu_process_callbacks+0x1ef/0x20d
(XEN) [  121.236758]    [<ffff82d0802425a5>] F 
common/softirq.c#__do_softirq+0x85/0x90
(XEN) [  121.244378]    [<ffff82d0802425e5>] F 
process_pending_softirqs+0x35/0x37
(XEN) [  121.251568]    [<ffff82d08022b91b>] F rcu_barrier+0x58/0x6e
(XEN) [  121.257639]    [<ffff82d0802addb1>] F cpu_down_helper+0x11/0x32
(XEN) [  121.264051]    [<ffff82d0802ae086>] F 
arch/x86/sysctl.c#smt_up_down_helper+0x1d6/0x1fe
(XEN) [  121.272454]    [<ffff82d08020878d>] F 
common/domain.c#continue_hypercall_tasklet_handler+0x54/0xb8
(XEN) [  121.281900]    [<ffff82d0802454e6>] F 
common/tasklet.c#do_tasklet_work+0x81/0xb4
(XEN) [  121.289786]    [<ffff82d080245803>] F do_tasklet+0x58/0x85
(XEN) [  121.295771]    [<ffff82d08027a0b4>] F 
arch/x86/domain.c#idle_loop+0x87/0xcb

So it's not in get_cpu_maps() loop. It seems to me it's not entering time sync 
for some
reason.

Igor

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