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From:   Roman Gushchin <guro@fb.com>
To:     Bruce Ashfield <bruce.ashfield@gmail.com>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tj@kernel.org" <tj@kernel.org>,
        Richard Purdie <richard.purdie@linuxfoundation.org>,
        "oleg@redhat.com" <oleg@redhat.com>
Subject: Re: ptrace/strace and freezer oddities and v5.2+ kernels
Thread-Topic: ptrace/strace and freezer oddities and v5.2+ kernels
Thread-Index: AQHVeHNRdeEItaYf7UK2FXG2TrAmk6dGmZIA
Date:   Wed, 2 Oct 2019 02:01:06 +0000
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On Tue, Oct 01, 2019 at 12:14:18PM -0400, Bruce Ashfield wrote:
> Hi all,
>

Hi Bruce!

> The Yocto project has an upcoming release this fall, and I've been trying=
 to
> sort through some issues that are happening with kernel 5.2+ .. although
> there is a specific yocto kernel, I'm testing and seeing this with
> normal / vanilla
> mainline kernels as well.
>=20
> I'm running into an issue that is *very* similar to the one discussed in =
the
> [REGRESSION] ptrace broken from "cgroup: cgroup v2 freezer" (76f969e)
> thread from this past may: https://lkml.org/lkml/2019/5/12/272
>=20
> I can confirm that I have the proposed fix for the initial regression rep=
ort in
> my build (05b2892637 [signal: unconditionally leave the frozen state
> in ptrace_stop()]),
> but yet I'm still seeing 3 or 4 minute runtimes on a test that used to ta=
ke 3 or
> 4 seconds.

So, the problem is that you're experiencing a severe performance regression
in some test, right?

>=20
> This isn't my normal area of kernel hacking, so I've so far come up empty
> at either fixing it myself, or figuring out a viable workaround. (well, I=
 can
> "fix" it by remove the cgroup_enter_frozen() call in ptrace_stop ...
> but obviously,
> that is just me trying to figure out what could be causing the issue).
>=20
> As part of the release, we run tests that come with various applications.=
 The
> ptrace test that is causing us issues can be boiled down to this:
>=20
> $ cd /usr/lib/strace/ptest/tests
> $ time ../strace -o log -qq -esignal=3Dnone -e/clock ./printpath-umovestr=
>ttt
>=20
> (I can provide as many details as needed, but I wanted to keep this initi=
al
> email relatively short).
>=20
> I'll continue to debug and attempt to fix this myself, but I grabbed the
> email list from the regression report in May to see if anyone has any ide=
as
> or angles that I haven't covered in my search for a fix.

I'm definitely happy to help, but it's a bit hard to say anything from what
you've provided. I'm not aware of any open issues with the freezer except
some spurious cgroup frozen<->not frozen transitions which can happen in so=
me
cases. If you'll describe how can I reproduce the issue, and I'll try to ta=
ke
a look asap.

Roman