linux-kernel - Re: [Question] How does perf still record the stack of a specified pid even when that process is interrupted and CPU is scheduled to other process

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Message-ID: <5549941C.6040901@huawei.com>
Date:	Wed, 6 May 2015 12:10:04 +0800
From:	Yunlong Song <yunlong.song@...wei.com>
To:	David Ahern <dsahern@...il.com>, <a.p.zijlstra@...llo.nl>,
	<paulus@...ba.org>, <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	<linux-kernel@...r.kernel.org>, <wangnan0@...wei.com>
Subject: Re: [Question] How does perf still record the stack of a specified
 pid even when that process is interrupted and CPU is scheduled to other process

On 2015/4/25 23:53, David Ahern wrote:
> On 4/25/15 8:05 AM, Yunlong Song wrote:
>> On 2015/4/24 21:58, David Ahern wrote:
>>> On 4/24/15 7:31 AM, Yunlong Song wrote:
>>>> Now we are profiling the performance of ext4 and f2fs on an eMMC card with iozone,
>>>> we find a case that ext4 is better than f2fs in random write under the test of
>>>> "iozone -s 262144 -r 64 -i 0 -i 2". We want to analyze the I/O delay of the two
>>>> file systems. We have got a conclusion that 1% of sys_write takes up 60% time of
>>>> the overall sys_write (262144/64=4096). We want to find out the call stack during
>>>> this specific 1% sys_write. Our idea is to record the stack in a certain time period
>>>> and since the specific 1% case takes up 60% time, the total number of records of its
>>>> stack should also takes up 60% of the total records, then we can recognize those stacks
>>>> and figure out what the I/O stack of f2fs is doing in the 1% case.
>>>
>>> And to address this specific profiling problem have you tried:
>>>
>>>     perf trace record -- iozone ...
>>>     perf trace -i perf.data -S
>>>
>>>
>>>
>>>
>>
>> But this only shows the system call like strace, but we want the call stack of kernel functions
>> in fact.
>>
> 
> We haven't added the callchain option yet; on the to-do list.
> 
> perf trace record -g -- iozone ...
> perf trace -i perf.data -s
> --> summary of system calls, max/min/average times
> 
> perf trace -i perf.data --duration 10.0 -T
> --> note the timestamp where the write took a "long" time
> 
> perf script
> --> search down to *around* the time of interest; you want the syscall entry; timestamp is for exit
> 
> .
> 

Hi, David,

It's almost what we want, we are eager to see it can work as a callchain option, since it's really a useful
tool in analyzing latency of I/O performance in production case.

-- 
Thanks,
Yunlong Song

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