linux-kernel - [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: <553A45CA.8020808@huawei.com>
Date:	Fri, 24 Apr 2015 21:31:54 +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: [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

[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], 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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