Date:	Tue, 13 Nov 2007 15:41:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Florian Boelstler <kernel@...lstler.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Strange delays / what usually happens every 10 min?


On Tue, 2007-11-13 at 15:24 +0100, Florian Boelstler wrote:
> Hi,
> 
> this issue has been already discussed on the kernelnewbies mailing list 
> [1],[2] and suggested to be further discussed here.
> 
> I am currently working on a MPC8540-based custom board, which runs Linux
> 2.6.15 (arch/ppc). The original Linux sources have been modified to 
> support that custom board. (Additional patches to support LTT are 
> applied as well, though disabled in the running kernel)
> 
> I set up a periodically running kernel thread, which is delayed for a
> single jiffy using schedule_timeout() in an infinite loop. It is used to
> measure delays between invocations of that thread. For measuring the
> distance in time the PPC's time base lower half register is used
> (obtained using get_cycles() defined in asm/timex.h).
> 
> The thread calculates the delay to the previous run and only outputs the
> result if a new maximum value has been determined (in respect to all
> previous cycles). Further the thread outputs a warning if a very "high"
> delay was determined. I.e. a delay greater than 5ms.
> 
> While running that test driver a delay of about 10ms _exactly_ occurs
> every 10 minutes.
> 
> The kernel is configured using CONFIG_HZ=1000 and CONFIG_PREEMPT.
> The CCB is at 333MHz, whereas the TBR update rate is 333 MHz / 8, i.e.
> 41,625 MHz.
> Kernel configuration as a whole is found here: 
> http://nopaste.info/5e4d0283bb.html
> 
> And now the funny part starts.
> I got a response from Bruce Rowen on kernelnewbies, telling me that he 
> came across the same problem. He increased his AMD-Geode-based platform 
> to 1GB of RAM (256MB before) and also hit the 10-minutes-issue a few 
> month ago (using Linux 2.6.13).
> Going back to 256MB cured the problem. I did the same thing by 
> instructing the boot loader in order to only use 256 MB of RAM (instead 
> of 512MB) and yes, the 10-minutes-issue was gone as well.
> 
> Apart of some kernel threads almost all user processes have been killed
> during the test. Only SSH and a bash were running (whereas a test with 
> network interfaces completely disabled and only operated from a serial 
> console turned out the same results).
> The kernel comes with compiled in CIFS support, some kernel debugging
> features like soft-lockup detection and preemption debugging. I.e. ps
> lists the kernel threads ksoftirqd, watchdog, events, khelper, kthread,
> kblockd, pdflush, aio, cifsoplockd and cifsdnotifyd.
> 
> An appropriate userspace test tool based on nanosleep() determined the
> same results like the kernel thread:
> 
> root@...0:/# /tmp/wait.rt
> looping 1 milli seconds nanosleep ...
> 15:26:16: #1 FRAME MAX 1996 us (at 4139773004 ticks)
> 15:26:16: #2 FRAME MAX 2002 us (at 4139856360 ticks)
> 15:26:16: #155 FRAME MAX 2102 us (at 4152597854 ticks)
> 15:41:37: #460398 FRAME MAX 8941 us (at 3813406605 ticks)
> 15:41:37: #460398 FRAME HIGH 8941 us (at 3813406605 ticks)
> 15:51:37: #760394 FRAME MAX 9936 us (at 3018602602 ticks)
> 15:51:37: #760394 FRAME HIGH 9936 us (at 3018602602 ticks)
> 16:01:37: #1060390 FRAME HIGH 9935 us (at 2223798809 ticks)
> 16:11:37: #1360386 FRAME HIGH 9934 us (at 1428994989 ticks)
> 16:21:37: #1660382 FRAME HIGH 9935 us (at 634191241 ticks)
> [...]

You could try the latency tracer from -rt.

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