Re: ptrace/strace and freezer oddities and v5.2+ kernels

[Bruce Ashfield]

On Tue, Oct 1, 2019 at 10:01 PM Roman Gushchin <guro@xxxxxx> wrote:
>
> 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.
> >
> > 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
> >
> > I can confirm that I have the proposed fix for the initial regression report 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 take 3 or
> > 4 seconds.
>
> So, the problem is that you're experiencing a severe performance regression
> in some test, right?

Hi Roman,

Correct. In particular, running some of the tests that ship with strace itself.
The performance change is so drastic, that it definitely makes you wonder
"What have I done wrong? Since everyone must be seeing this" .. and I
always blame myself first.

>
> >
> > 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).
> >
> > 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:
> >
> > $ cd /usr/lib/strace/ptest/tests
> > $ time ../strace -o log -qq -esignal=none -e/clock ./printpath-umovestr>ttt
> >
> > (I can provide as many details as needed, but I wanted to keep this initial
> > email relatively short).
> >
> > 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 ideas
> > 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 some
> cases. If you'll describe how can I reproduce the issue, and I'll try to take
> a look asap.

That would be great.

I'll attempt to remove all of the build system specifics out of this
(and Richard Purdie
on the cc' of this can probably help provide more details / setup info as well).

We are running the built-in tests of strace. So here's a cut and paste of what I
did to get the tests available (ignore/skip what is common sense or isn't needed
in your test rig).

% git clone https://github.com/strace/strace.git
% cd strace
% ./bootstrap
# the --enable flag isn't strictly required, but may break on some
build machines
% ./configure --enable-mpers=no
% make
% make check-TESTS

That last step will not only build the tests, but run them all .. so
^c the run once
it starts, since it is a lot of noise (we carry a patch to strace that
allows us to build
the tests without running them).

% cd tests
% time strace -o log -qq -esignal=none -e/clock ./printpath-umovestr > fff
real    0m2.566s
user    0m0.284s
sys     0m2.519

On pre-cgroup2 freezer kernels, you see a run time similar to what I have above.
On the newer kernels we are testing, it is taking 3 or 4 minutes to
run the test.

I hope that is simple enough to setup and try. Since I've been seeing
this on both
mainline kernels and the yocto reference kernels, I don't think it is
something that
I'm carrying in the distro/reference kernel that is causing this (but
again, I always
blame myself first). If you don't see that same run time, then that
does point the finger
back at what we are doing and I'll have to apologize for chewing up some of your
time.

Cheers,

Bruce


>
> Roman



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