Re: Fwd: filtering perf itself

[Brendan Gregg <brendan.d.gregg@gmail.com>]

On Mon, Jul 7, 2014 at 12:00 PM, Brendan Gregg
<brendan.d.gregg@gmail.com> wrote:
> On Mon, Jul 7, 2014 at 11:44 AM, David Ahern <dsahern@gmail.com> wrote:
>> On 7/7/14, 12:38 PM, Brendan Gregg wrote:
>>>
>>> G'Day perf users,
>>>
>>> Is there a way to filter perf from tracing itself?
>>>
>>> Here's an idle system:
>>>
>>> # ./perf record -e syscalls:sys_enter_read -a sleep 5
>>> [ perf record: Woken up 2 times to write data ]
>>> [ perf record: Captured and wrote 0.569 MB perf.data (~24864 samples) ]
>>> # ./perf record -e syscalls:sys_enter_write -a sleep 5
>>> [ perf record: Woken up 0 times to write data ]
>>> [ perf record: Captured and wrote 150.381 MB perf.data (~6570251 samples)
>>> ]
>>>
>>> Note the disparity. perf is capturing its own writes, creating a feedback
>>> loop.
>>
>>
>> Not a filter, but works around the problem using mmap'ed output file:
>>
>> https://github.com/dsahern/linux/commit/ae2d7010256f5a5b247fb4df9f764a911a34a2f3
>>
>
> Ah, thanks David, that should work! Looking forward to having this
> patch included.
>

While this should help a lot of cases, I realized I am using stdout
from time to time as well (similar to the "perf script" framework),
where I don't think mmap() is going to work. Eg:

# perf record -e raw_syscalls:sys_enter -a -o - sleep 5 | perf script
-i - | stuff...

Maybe perf should skip itself by-default, unless asked (eg, -I to
include perf's own events). Or some way to filter it would also work,
eg, common_ppid.

Brendan
-- 
http://www.brendangregg.com