linux-kernel - Re: Athlon64x2 problem with 2.6.18-rt4 and hrtimers

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Message-ID: <451AFBE5.9000008@redhat.com>
Date:	Wed, 27 Sep 2006 17:32:05 -0500
From:	Clark Williams <williams@...hat.com>
To:	tglx@...utronix.de
CC:	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Athlon64x2 problem with 2.6.18-rt4 and hrtimers

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Thomas Gleixner wrote:
> Clark,
> 
> On Wed, 2006-09-27 at 15:30 -0500, Clark Williams wrote:
>> What I've been seeing were some very long latencies while running
>> cyclictimer (v0.1 thru v0.9). The test would start up and run with quite
>> good numbers for a few thousand iterations, then really large latencies
>> would start to occur (between 5000 and 30000 us). I have not dumped the
>> stats to a file and done any statistical analysis as to how frequent the
>> large latencies are, nor whether they're trending up or down.
> 
> Would be interesting to see.

I'll email you two files of 30k loops each, one from cpu0 and the other
bound to cpu1. Command line will look like this:

$ sudo taskset 0x1 cyclictest --prio=1 --loops=30000 --verbose >cpu0.out
$ sudo taskset 0x2 cyclictest --prio=1 --loops=30000 --verbose >cpu1.out

> 
>> I had a suspicion that it involved signal delivery because of the following:
>>
>> $ sudo ./cyclictest32 --prio=1
>> 0.01 0.03 0.01 1/151 3486
>>
>> T: 0 ( 3484) P: 1 I:    1000 C:    8955 Min:      10 Act:   38011 Avg:
>>  20919 Max:   38445
>> $ sudo ./cyclictest32 --prio=1 --nanosleep
>> 0.01 0.03 0.00 1/151 3490
>>
>> T: 0 ( 3488) P: 1 I:    1000 C:   12995 Min:       6 Act:       7 Avg:
>>      6 Max:      31
>>
>> As you can see, the Avg and Max times stay quite low when using
>> nanosleep; the latencies only happen when using itimers.
> 
> Also can you run a test with prio=80, if there is any difference ?

No difference (wouldn't expect there to be).

> 
>> I was describing this behavior when DJ Delorie suggested that it might
>> be affinity based rather than signal delivery. He suggested binding the
>> test to a particular processor, so I did with the following results:
>>
>> $ sudo taskset 0x1 ./cyclictest32 --prio=1
>> 0.11 0.07 0.08 1/149 3311
>>
>> T: 0 ( 3305) P: 1 I:    1000 C:   45709 Min:       8 Act:      11 Avg:
>>     10 Max:      53
>> $ sudo taskset 0x2 ./cyclictest32 --prio=1
>> 0.03 0.05 0.08 1/149 3323
>>
>> T: 0 ( 3313) P: 1 I:    1000 C:   74451 Min:      10 Act:   58011 Avg:
>>  28976 Max:   58818
>>
>> So it seems that the latencies only occur on processor 1 (not on
>> processor 0).  I booted 2.6.18-rt4 with and without idle=poll (as Ingo
>> suggested) and saw the long latencies in both cases when the test was
>> bound to processor 1 (never with processor 0).
> 
> Hmm. 

Took the words right out of my mouth :)

> 
>> Hopefully this is useful information. I just wanted to let you guys
>> know, since you'll probably have a fix available long before I can
>> comprehend the arch/x86_64 code where this probably occurs.  Let me know
>> if you want other things, like the .config or something else.
> 
> Can you please switch on CONFIG_LATENCY_TRACE (depends on
> CONFIG_LATENCY_TIMING) ?
> 
> Use the latest version of cyclictest and add -b XXX to the command line,
> where XXX is the maximum latency in micro seconds. Once the latency is
> greater than the given maximum, the kernel tracer and cyclictest is
> stopped.
> 
> Now you can read the kernel trace:
> 
> cat /proc/latency_trace >trace.log 
> 
> The trace should give us more insight.
> 
> Please be aware that the tracer adds significant overhead to the kernel,
> so the latencies will be much higher.
> 

Yeah, I've used the latency tracer before.  I'll do this in the morning
when I get in.

Thanks,

Clark

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