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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Date:	Fri, 24 Apr 2015 21:49:03 +0800
From:	Yunlong Song <yunlong.song@...wei.com>
To:	<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 2015/4/24 21:31, Yunlong Song wrote:
> [Profiling Background]
> 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.
> 
> [Profiling Problem]
> 
> Although perf can record the events (with call stack) of a specified pid, e.g. using
> "perf record -g iozone -s 262144 -r 64 -i 0 -i 2". But we find iozone is interrupted
> and the CPU is scheduled to other process. As a result, perf will not record any events
> of iozone until iozone's context is recovered and the CPU is scheduled to continue
> processing the sys_write of iozone. 

>This obeys our initial idea which is described in [Profiling Background],

This "disobeys" our initial idea which is described in [Profiling Background],

 since we cannot recognize the call stacks of the specific 1% case
> by using the ratio of the record number.
> 
> [Alternative Solution without Perf]
> We can use /proc/#pid/stack to record the stack in a certain period (e.g. 1ms) of iozone,
> no matter whether iozone is interrupted or not. However, we have not taken a deep sight
> into this, since we want to use perf to do this kind of thing.
> 
> [Question about Perf]
> So we have a question that "How does perf still record the stack of a specified pid even
> when that process is interrupted and CPU is scheduled to other process?"
> 


-- 
Thanks,
Yunlong Song

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