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: <553BB895.20800@gmail.com>
Date:	Sat, 25 Apr 2015 09:53:57 -0600
From:	David Ahern <dsahern@...il.com>
To:	Yunlong Song <yunlong.song@...wei.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 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
--
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