Re: 2.6.18-rc1-mm1 - VPN chewing CPU like crazy..

[Andrew Morton]

(cc's added)

On Tue, 11 Jul 2006 00:20:22 -0400
Valdis.Kletnieks@xxxxxx wrote:

> (This is *NOT* new with 2.6.18-rc1-mm1, I've seen it a few times before, no
> idea when it started... I think I've seen it as far back as 2.6.16-mm2 or so).
> 
> Most of the time, the VPN comes up just fine.
> 
> Jul 10 22:44:28 turing-police kernel: [ 7180.696000] CSLIP: code copyright 1989 Regents of the University of California
> Jul 10 22:44:28 turing-police kernel: [ 7180.701000] PPP generic driver version 2.4.2
> Jul 10 23:00:26 turing-police kernel: [ 8137.903000] PPP MPPE Compression module registered
> 
> However, sometimes (usually when restarting after the VPN gets dropped due to
> a network burp, etc), it will start up, get connected, and then the the first
> (or one of the first) data packets over the VPN will suddenly spike the
> CPU to 100% (about 50% userspace and 50% kernel).  A quick oprofile check
> shows the kernel CPU getting sucked:
> 
> samples  %        image name               app name                 symbol name
> 10901    18.2805  arc4.ko                  arc4                     .text
> 5046      8.4619  ppp_async.ko             ppp_async                ppp_async_push
> 4865      8.1584  pptp                     pptp                     (no symbols)
> 4382      7.3484  arc4.ko                  arc4                     arc4_set_key
> 4331      7.2629  vmlinux                  vmlinux                  sha_transform
> 4203      7.0482  libfb.so                 libfb.so                 fbCopyAreammx
> 3342      5.6044  vmlinux                  vmlinux                  ecb_process
> 1324      2.2203  libgdk_imlib.so.1.9.13   libgdk_imlib.so.1.9.13   (no symbols)
> 1149      1.9268  ip_tables.ko             ip_tables                ipt_do_table
> 1071      1.7960  vmlinux                  vmlinux                  local_bh_enable
> 848       1.4221  vmlinux                  vmlinux                  acpi_pm_read
> 816       1.3684  vmlinux                  vmlinux                  __local_bh_disable
> 698       1.1705  vmlinux                  vmlinux                  n_tty_receive_buf
> 651       1.0917  vmlinux                  vmlinux                  do_page_fault
> 
> (pptp is the userspace control process)
> 
> gkrellm reports that about 4 mbytes/sec is being sent on ppp0 - but shows
> zero traffic on eth0 (the underlying interface).  When working properly,
> both ppp0 and eth0 show the traffic.
> 
> This continues until I get fed up and manually take the VPN down, at which
> point it happily gets rid of ppp0 and CPU load returns to normal.
> 
> I haven't found a clean way to reproduce it - sometimes I won't see it for
> 2-3 weeks, then it will pop up several times in an evening.
> 
> Any suggestions/hints (besides rebuilding the implicated .ko with debugging
> symbols so oprofile can be more granular - that's already on the to-do list)?
> 

I'd suggest you whack sysrq-T 5-10 times when it happens, capture a few
stack traces.

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