[ovs-dev] 10-25 packet drops every few (10-50) seconds TCP (iperf3)

Shahaji Bhosle shahaji.bhosle at broadcom.com
Tue Jun 30 16:56:27 UTC 2020


Thanks Flavio,
Are there any special requirements for RCU on ARM vs x86.

I am following what the above document is saying...Do you think I need to
do something more than the below?
Thanks again and appreciate the help. Shahaji

1. Isolate the CPU cores
*isolcpus=1,2,3,4,5,6,7 nohz_full=1-7 rcu_nocbs=1-7*
2. Setting CONFIG_NO_HZ_FULL=y
root at bcm958802a8046c:~/vinay_rx/dynticks-testing# zcat /proc/config.gz
|grep HZ
CONFIG_NO_HZ_COMMON=y
# CONFIG_HZ_PERIODIC is not set
# CONFIG_NO_HZ_IDLE is not set
*CONFIG_NO_HZ_FULL*=y
# CONFIG_NO_HZ_FULL_ALL is not set
# CONFIG_NO_HZ is not set
# CONFIG_HZ_100 is not set
CONFIG_HZ_250=y
# CONFIG_HZ_300 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=250



On Tue, Jun 30, 2020 at 12:50 PM Flavio Leitner <fbl at sysclose.org> wrote:

>
> Right, you might want to review Documentation/timers/no_hz.rst from
> the kernel sources and look for RCU implications section where
> it explains how to move RCU callbacks.
>
> fbl
>
> On Tue, Jun 30, 2020 at 12:08:05PM -0400, Shahaji Bhosle wrote:
> > Hi Flavio,
> > I wrote a small program which has do_nothing for loop and I measure the
> > timestamps across the do nothing loop. I am seeing 3% of the time around
> > the 1 second mark when the arch_timer fires I get the timestamps to be
> off
> > by 25% of the exprected value. I ran trace-cmd to see what is going on
> and
> > see the below. Looks like some issue with *gic_handle_irg*(), not seeing
> > tihs behaviour on x86 host, something special with ARM v8.
> > Thanks, Shahaji
> >
> >   %21.77  (14181) arm_stb_user_lo                    rcu_dyntick #922
> >          |
> >          --- *rcu_dyntick*
> >             |
> >             |--%46.85-- gic_handle_irq  # 432
> >             |
> >             |--%23.32-- context_tracking_user_exit  # 215
> >             |
> >             |--%22.34-- context_tracking_user_enter  # 206
> >             |
> >             |--%2.60-- SyS_execve  # 24
> >             |
> >             |--%1.30-- do_page_fault  # 12
> >             |
> >             |--%0.65-- SyS_write  # 6
> >             |
> >             |--%0.65-- schedule  # 6
> >             |
> >             |--%0.65-- SyS_nanosleep  # 6
> >             |
> >             |--%0.65-- syscall_trace_enter  # 6
> >             |
> >             |--%0.65-- SyS_faccessat  # 6
> >
> >   %5.01  (14181) arm_stb_user_lo                rcu_utilization #212
> >          |
> >          --- *rcu_utilization*
> >             |
> >             |--%96.23-- gic_handle_irq  # 204
> >             |
> >             |--%1.89-- SyS_nanosleep  # 4
> >             |
> >             |--%0.94-- SyS_exit_group  # 2
> >             |
> >             |--%0.94-- do_notify_resume  # 2
> >
> >   %4.86  (14181) arm_stb_user_lo                      user_exit #206
> >          |
> >          --- *user_exit*
> >           context_tracking_user_exit
> >
> >   %4.86  (14181) arm_stb_user_lo     context_tracking_user_exit #206
> >          |
> >          --- context_tracking_user_exit
> >
> >   %4.86  (14181) arm_stb_user_lo    context_tracking_user_enter #206
> >          |
> >          --- context_tracking_user_enter
> >
> >   %4.86  (14181) arm_stb_user_lo                     user_enter #206
> >          |
> >          --- *user_enter*
> >           context_tracking_user_enter
> >
> >   %2.95  (14181) arm_stb_user_lo                 gic_handle_irq #125
> >          |
> >          --- gic_handle_irq
> >
> >
> > On Tue, Jun 30, 2020 at 9:45 AM Flavio Leitner <fbl at sysclose.org> wrote:
> >
> > > On Tue, Jun 02, 2020 at 12:56:51PM -0700, Vinay Gupta wrote:
> > > > Hi Flavio,
> > > >
> > > > Thanks for your reply.
> > > > I have captured the suggested information but do not see anything
> that
> > > > could cause the packet drops.
> > > > Can you please take a look at the below data and see if you can find
> > > > something unusual ?
> > > > The PMDs are running on CPU 1,2,3,4 and CPU 1-7 are isolated cores.
> > > >
> > > >
> > >
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> > > > root at bcm958802a8046c:~# cstats ; sleep 10; cycles
> > > > pmd thread numa_id 0 core_id 1:
> > > >   idle cycles: 99140849 (7.93%)
> > > >   processing cycles: 1151423715 (92.07%)
> > > >   avg cycles per packet: 116.94 (1250564564/10693918)
> > > >   avg processing cycles per packet: 107.67 (1151423715/10693918)
> > > > pmd thread numa_id 0 core_id 2:
> > > >   idle cycles: 118373662 (9.47%)
> > > >   processing cycles: 1132193442 (90.53%)
> > > >   avg cycles per packet: 124.39 (1250567104/10053309)
> > > >   avg processing cycles per packet: 112.62 (1132193442/10053309)
> > > > pmd thread numa_id 0 core_id 3:
> > > >   idle cycles: 53805933 (4.30%)
> > > >   processing cycles: 1196762002 (95.70%)
> > > >   avg cycles per packet: 107.35 (1250567935/11649948)
> > > >   avg processing cycles per packet: 102.73 (1196762002/11649948)
> > > > pmd thread numa_id 0 core_id 4:
> > > >   idle cycles: 189102938 (15.12%)
> > > >   processing cycles: 1061463293 (84.88%)
> > > >   avg cycles per packet: 143.47 (1250566231/8716828)
> > > >   avg processing cycles per packet: 121.77 (1061463293/8716828)
> > > > pmd thread numa_id 0 core_id 5:
> > > > pmd thread numa_id 0 core_id 6:
> > > > pmd thread numa_id 0 core_id 7:
> > >
> > >
> > > The core_id 3 is high loaded, and then it's more likely to show
> > > the drop issue when some other event happens.
> > >
> > > I think you need to run perf as I recommended before and see if
> > > there are context switches happening and why they are happening.
> > >
> > > If a context switch happens, it's either because the core is not
> > > well isolated or some other thing is going on. It will help to
> > > understand why the queue wasn't serviced for a certain amount of
> > > time.
> > >
> > > The issue is that running perf might introduce some load, so you
> > > will need adjust the traffic rate accordingly.
> > >
> > > HTH,
> > > fbl
> > >
> > >
> > >
> > > >
> > > >
> > > > *Runtime summary*                          comm  parent   sched-in
> > > > run-time    min-run     avg-run     max-run  stddev  migrations
> > > >                                           (count)       (msec)
>  (msec)
> > > >    (msec)      (msec)       %
> > > >
> > >
> ---------------------------------------------------------------------------------------------------------------------
> > > >                 ksoftirqd/0[7]       2          1        0.079
> 0.079
> > > >     0.079       0.079    0.00       0
> > > >                   rcu_sched[8]       2         14        0.067
> 0.002
> > > >     0.004       0.009    9.96       0
> > > >                    rcuos/4[38]       2          6        0.027
> 0.002
> > > >     0.004       0.008   20.97       0
> > > >                    rcuos/5[45]       2          4        0.018
> 0.004
> > > >     0.004       0.005    6.63       0
> > > >                kworker/0:1[71]       2         12        0.156
> 0.008
> > > >     0.013       0.019    6.72       0
> > > >                  mmcqd/0[1230]       2          3        0.054
> 0.001
> > > >     0.018       0.031   47.29       0
> > > >             kworker/0:1H[1248]       2          1        0.006
> 0.006
> > > >     0.006       0.006    0.00       0
> > > >            kworker/u16:2[1547]       2         16        0.045
> 0.001
> > > >     0.002       0.012   26.19       0
> > > >                     ntpd[5282]       1          1        0.063
> 0.063
> > > >     0.063       0.063    0.00       0
> > > >                 watchdog[6988]       1          2        0.089
> 0.012
> > > >     0.044       0.076   72.26       0
> > > >             ovs-vswitchd[9239]       1          2        0.326
> 0.152
> > > >     0.163       0.173    6.45       0
> > > >        revalidator8[9309/9239]    9239          2        1.260
> 0.607
> > > >     0.630       0.652    3.58       0
> > > >                    perf[27150]   27140          1        0.000
> 0.000
> > > >     0.000       0.000    0.00       0
> > > >
> > > > Terminated tasks:
> > > >                   sleep[27151]   27150          4        1.002
> 0.015
> > > >     0.250       0.677   58.22       0
> > > >
> > > > Idle stats:
> > > >     CPU  0 idle for    999.814  msec  ( 99.84%)
> > > >
> > > >
> > > >
> > > > *CPU  1 idle entire time window    CPU  2 idle entire time window
> > > CPU  3
> > > > idle entire time window    CPU  4 idle entire time window*
> > > >     CPU  5 idle for    500.326  msec  ( 49.96%)
> > > >     CPU  6 idle entire time window
> > > >     CPU  7 idle entire time window
> > > >
> > > >     Total number of unique tasks: 14
> > > > Total number of context switches: 115
> > > >            Total run time (msec):  3.198
> > > >     Total scheduling time (msec): 1001.425  (x 8)
> > > > (END)
> > > >
> > > >
> > > >
> > > > *02:16:22      UID      TGID       TID    %usr %system  %guest
>  %wait
> > > >  %CPU   CPU  Command *02:16:23        0      9239         -  100.00
> > > 0.00
> > > >    0.00    0.00  100.00     5  ovs-vswitchd
> > > > 02:16:23        0         -      9239    2.00    0.00    0.00    0.00
> > > >  2.00     5  |__ovs-vswitchd
> > > > 02:16:23        0         -      9240    0.00    0.00    0.00    0.00
> > > >  0.00     0  |__vfio-sync
> > > > 02:16:23        0         -      9241    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__eal-intr-thread
> > > > 02:16:23        0         -      9242    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__dpdk_watchdog1
> > > > 02:16:23        0         -      9244    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__urcu2
> > > > 02:16:23        0         -      9279    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__ct_clean3
> > > > 02:16:23        0         -      9308    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__handler9
> > > > 02:16:23        0         -      9309    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__revalidator8
> > > > 02:16:23        0         -      9328    0.00    0.00    0.00    0.00
> > > >  0.00     6  |__pmd13
> > > > 02:16:23        0         -      9330  100.00    0.00    0.00    0.00
> > > >  100.00     3  |__pmd12
> > > > 02:16:23        0         -      9331  100.00    0.00    0.00    0.00
> > > >  100.00     1  |__pmd11
> > > > 02:16:23        0         -      9332    0.00    0.00    0.00    0.00
> > > >  0.00     7  |__pmd10
> > > > 02:16:23        0         -      9333    0.00    0.00    0.00    0.00
> > > >  0.00     5  |__pmd16
> > > > 02:16:23        0         -      9334  100.00    0.00    0.00    0.00
> > > >  100.00     2  |__pmd15
> > > > 02:16:23        0         -      9335  100.00    0.00    0.00    0.00
> > > >  100.00     4  |__pmd14
> > > >
> > >
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> > > >
> > > > Thanks
> > > > Vinay
> > > >
> > > > On Tue, Jun 2, 2020 at 12:06 PM Flavio Leitner <fbl at sysclose.org>
> wrote:
> > > >
> > > > > On Mon, Jun 01, 2020 at 07:27:09PM -0400, Shahaji Bhosle via dev
> wrote:
> > > > > > Hi Ben/Ilya,
> > > > > > Hope you guys are doing well and staying safe. I have been
> chasing a
> > > > > weird
> > > > > > problem with small drops and I think that is causing lots of TCP
> > > > > > retransmission.
> > > > > >
> > > > > > Setup details
> > > > > > iPerf3(1k-5K
> > > > > > Servers)<--DPDK2:OvS+DPDK(VxLAN:BOND)[DPDK0+DPDK1)<====2x25G<====
> > > > > > [DPDK0+DPDK1)(VxLAN:BOND)OVS+DPDKDPDK2<---iPerf3(Clients)
> > > > > >
> > > > > > All the Drops are ring drops on BONDed functions on the server
> > > side.  I
> > > > > > have 4 CPUs each with 3PMD threads, DPDK0, DPDK1 and DPDK2 all
> > > running
> > > > > with
> > > > > > 4 Rx rings each.
> > > > > >
> > > > > > What is interesting is when I give each Rx rings its own CPU the
> > > drops go
> > > > > > away. Or if I set cother_config:emc-insert-inv-prob=1 the drops
> go
> > > away.
> > > > > > But I need to scale up the number of flows so trying to run this
> > > with EMC
> > > > > > disabled.
> > > > > >
> > > > > > I can tell that the rings are not getting serviced for 30-40usec
> > > because
> > > > > of
> > > > > > some kind context switch or interrupts on these cores. I have
> tried
> > > to do
> > > > > > the usual isolation, nohz_full rcu_nocbs etc. Move all the
> interrupts
> > > > > away
> > > > > > from these cores etc. But nothing helps. I mean it improves, but
> the
> > > > > drops
> > > > > > still happen.
> > > > >
> > > > > When you disable the EMC (or reduce its efficiency) the per packet
> cost
> > > > > increases, then it becomes more sensitive to variations. If you
> share
> > > > > a CPU with multiple queues, you decrease the amount of time
> available
> > > > > to process the queue. In either case, there will be less room to
> > > tolerate
> > > > > variations.
> > > > >
> > > > > Well, you might want to use 'perf' and monitor for the scheduling
> > > events
> > > > > and then based on the stack trace see what is causing it and try to
> > > > > prevent it.
> > > > >
> > > > > For example:
> > > > > # perf record -e sched:sched_switch -a -g sleep 1
> > > > >
> > > > > For instance, you might see that another NIC used for management
> has
> > > > > IRQs assigned to one isolated CPU. You can move it to another CPU
> to
> > > > > reduce the noise, etc...
> > > > >
> > > > > Another suggestion is look at PMD thread idle statistics because it
> > > > > will tell you how much "extra" room you have left. As it approaches
> > > > > to 0, more fine tuned your setup needs to be to avoid drops.
> > > > >
> > > > > HTH,
> > > > > --
> > > > > fbl
> > > > >
> > >
> > > --
> > > fbl
> > >
>
> --
> fbl
>


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