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

[Jürgen Groß <jgross@xxxxxxxx>]

On 22.02.20 17:42, Igor Druzhinin wrote:
> 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.
Interesting. Looking further into that.

At least time_calibration() is missing to call get_cpu_maps().


Juergen

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