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

From: David Ahern
Date: Sat Apr 25 2015 - 11:54:08 EST

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 -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 -s
--> summary of system calls, max/min/average times

perf trace -i --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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