[RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Mathieu Desnoyers]

Hi,

This is a set of bugfix releases of the LTTng modules kernel tracer.
It covers the three currently active lttng-modules branches: the
2.9 and 2.10 stable branches, as well as the 2.11 branch in release
candidate cycle.

Those releases add support for kernel 4.19.

One important improvement is to prevent allocation of buffers larger
than the available memory, which can cause the OOM killer to trigger.
Even if the OOM killer end up having to trigger, the current OOM kill
target is set to the current thread while allocating buffers.

Enjoy!

Mathieu

Project website: http://lttng.org
Documentation: http://lttng.org/docs
Download link: http://lttng.org/download

Changelog:

2018-11-01 (National Men Make Dinner Day) LTTng modules 2.11.0-rc2
        * Fix: update kvm instrumentation for SLES12 SP2 LTSS >= 4.4.121-92.92
        * Fix: Add missing const to lttng_tracepoint_ptr_deref prototype
        * Fix: adapt to kernel relative references
        * Fix: implicit declarations caused by buffer size checks.
        * Prevent allocation of buffers if exceeding available memory
        * Fix: btrfs instrumentation namespacing
        * Fix: Convert rcu tracepointis to gp_seq (v4.19)
        * Fix: tracing: Centralize preemptirq tracepoints (4.19)
        * Fix: net: expose sk wmem in sock_exceed_buf_limit tracepoint (4.19)
        * Fix: access migrate_disable field directly
        * Fix: out of memory error handling

2018-11-01 (National Men Make Dinner Day) LTTng modules 2.10.8
        * Fix: update kvm instrumentation for SLES12 SP2 LTSS >= 4.4.121-92.92
        * Fix: Add missing const to lttng_tracepoint_ptr_deref prototype
        * Fix: adapt to kernel relative references
        * Fix: implicit declarations caused by buffer size checks.
        * Prevent allocation of buffers if exceeding available memory
        * Fix: Convert rcu tracepointis to gp_seq (v4.19)
        * Fix: net: expose sk wmem in sock_exceed_buf_limit tracepoint (4.19)
        * Fix: access migrate_disable field directly
        * Fix: out of memory error handling

2018-11-01 (National Men Make Dinner Day) LTTng modules 2.9.11
        * Fix: update kvm instrumentation for SLES12 SP2 LTSS >= 4.4.121-92.92
        * Fix: Add missing const to lttng_tracepoint_ptr_deref prototype
        * Fix: adapt to kernel relative references
        * Fix: implicit declarations caused by buffer size checks.
        * Prevent allocation of buffers if exceeding available memory
        * Fix: Convert rcu tracepointis to gp_seq (v4.19)
        * Fix: net: expose sk wmem in sock_exceed_buf_limit tracepoint (4.19)
        * Fix: access migrate_disable field directly
        * Fix: out of memory error handling


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [diamon-discuss] [RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Joel Fernandes]

On Thu, Nov 1, 2018 at 3:56 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> Hi,
>
> This is a set of bugfix releases of the LTTng modules kernel tracer.
> It covers the three currently active lttng-modules branches: the
> 2.9 and 2.10 stable branches, as well as the 2.11 branch in release
> candidate cycle.
>
> Those releases add support for kernel 4.19.
>
> One important improvement is to prevent allocation of buffers larger
> than the available memory, which can cause the OOM killer to trigger.
> Even if the OOM killer end up having to trigger, the current OOM kill
> target is set to the current thread while allocating buffers.

This is interesting. Me and Steve were looking at exactly this issue
with the ftrace ring buffer a few months ago. Turns out that even
setting the OOM kill target may not be enough to prevent all OOMs. I
don't remember the reason why not, I'll have to dig out those threads
but that's what the -mm folks said at the time. I did remember vaguely
that I tested it and the kill target doesn't always get killed.. its
possible that something *other* parallel allocation can be victimized
AFAIR, even though the culprit is the kill target.

 - Joel

Re: [diamon-discuss] [RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Mathieu Desnoyers]

----- On Nov 1, 2018, at 7:33 PM, Joel Fernandes via diamon-discuss diamon-discuss@lists.linuxfoundation.org wrote:

> On Thu, Nov 1, 2018 at 3:56 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> Hi,
>>
>> This is a set of bugfix releases of the LTTng modules kernel tracer.
>> It covers the three currently active lttng-modules branches: the
>> 2.9 and 2.10 stable branches, as well as the 2.11 branch in release
>> candidate cycle.
>>
>> Those releases add support for kernel 4.19.
>>
>> One important improvement is to prevent allocation of buffers larger
>> than the available memory, which can cause the OOM killer to trigger.
>> Even if the OOM killer end up having to trigger, the current OOM kill
>> target is set to the current thread while allocating buffers.
> 
> This is interesting. Me and Steve were looking at exactly this issue
> with the ftrace ring buffer a few months ago. Turns out that even
> setting the OOM kill target may not be enough to prevent all OOMs. I
> don't remember the reason why not, I'll have to dig out those threads
> but that's what the -mm folks said at the time. I did remember vaguely
> that I tested it and the kill target doesn't always get killed.. its
> possible that something *other* parallel allocation can be victimized
> AFAIR, even though the culprit is the kill target.
> 

Hi Joel,

Sorry for the late reply. Thanks for your input!

Here is a description of the solution we implemented:

"   Get an estimate of the number of available pages and return ENOMEM if
    there are not enough pages to cover the needs of the caller. Also, mark
    the calling user thread as the first target for the OOM killer in case
    the estimate of available pages was wrong.
    
    This greatly reduces the attack surface of this issue as well as reducing
    its potential impact.
    
    This approach is inspired by the one taken by the Linux kernel
    trace ring buffer[1]."

This is implemented in commit 1f0ab1eb040 "Prevent allocation of buffers if exceeding available memory"
within lttng-modules.

Are you aware of another way to achieve this that would prevent the incorrect
OOM victimization scenario you describe above ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [diamon-discuss] [RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Joel Fernandes]

On Tue, Mar 19, 2019 at 01:34:34PM -0400, Mathieu Desnoyers wrote:
> ----- On Nov 1, 2018, at 7:33 PM, Joel Fernandes via diamon-discuss diamon-discuss@lists.linuxfoundation.org wrote:
> 
> > On Thu, Nov 1, 2018 at 3:56 PM Mathieu Desnoyers
> > <mathieu.desnoyers@efficios.com> wrote:
> >>
> >> Hi,
> >>
> >> This is a set of bugfix releases of the LTTng modules kernel tracer.
> >> It covers the three currently active lttng-modules branches: the
> >> 2.9 and 2.10 stable branches, as well as the 2.11 branch in release
> >> candidate cycle.
> >>
> >> Those releases add support for kernel 4.19.
> >>
> >> One important improvement is to prevent allocation of buffers larger
> >> than the available memory, which can cause the OOM killer to trigger.
> >> Even if the OOM killer end up having to trigger, the current OOM kill
> >> target is set to the current thread while allocating buffers.
> > 
> > This is interesting. Me and Steve were looking at exactly this issue
> > with the ftrace ring buffer a few months ago. Turns out that even
> > setting the OOM kill target may not be enough to prevent all OOMs. I
> > don't remember the reason why not, I'll have to dig out those threads
> > but that's what the -mm folks said at the time. I did remember vaguely
> > that I tested it and the kill target doesn't always get killed.. its
> > possible that something *other* parallel allocation can be victimized
> > AFAIR, even though the culprit is the kill target.
> > 
> 
> Hi Joel,
> 
> Sorry for the late reply. Thanks for your input!

No problem, thanks for the reply :)

> Here is a description of the solution we implemented:
> 
> "   Get an estimate of the number of available pages and return ENOMEM if
>     there are not enough pages to cover the needs of the caller. Also, mark
>     the calling user thread as the first target for the OOM killer in case
>     the estimate of available pages was wrong.
>     
>     This greatly reduces the attack surface of this issue as well as reducing
>     its potential impact.
>     
>     This approach is inspired by the one taken by the Linux kernel
>     trace ring buffer[1]."
> 
> This is implemented in commit 1f0ab1eb040 "Prevent allocation of buffers if exceeding available memory"
> within lttng-modules.
> 
> Are you aware of another way to achieve this that would prevent the incorrect
> OOM victimization scenario you describe above ?

Adding Steve as well.

As far as I can see, lttng does exactly the same thing ftrace does, however
ftrace also does allocations with __GFP_RETRY_MAYFAIL. I think you want to do
that too. That will prevent the ring buffer allocation from being the source
of the OOM trigger. However, if OOM is triggered due to parallel allocations
from other sources, then the ring-buffer allocation will still be killed even
though it is not the only source of the large allocation.  That is the best
we came up with.

Also does it also make sense for lttng ring buffer to use the ftrace code for
ring buffer, or make the ftrace ring buffer better and have lttng use it? Or
is the lttng ring buffer design too radically different?

thanks,

 - Joel

Re: [diamon-discuss] [RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Steven Rostedt]

On Thu, 21 Mar 2019 08:41:22 -0400
Joel Fernandes <joel@joelfernandes.org> wrote:

> Also does it also make sense for lttng ring buffer to use the ftrace code for
> ring buffer, or make the ftrace ring buffer better and have lttng use it? Or
> is the lttng ring buffer design too radically different?

We tried in the past and never actually got something that we both
could agree on. Now I believe the user space tools depend on the way
each one is, so to change one will break the tools that read it.

-- Steve

Re: [diamon-discuss] [RELEASE] LTTng-modules 2.9.11, 2.10.8, 2.11.0-rc2 (Linux kernel tracer)

[Joel Fernandes]

On Thu, Mar 21, 2019 at 09:13:56AM -0400, Steven Rostedt wrote:
> On Thu, 21 Mar 2019 08:41:22 -0400
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > Also does it also make sense for lttng ring buffer to use the ftrace code for
> > ring buffer, or make the ftrace ring buffer better and have lttng use it? Or
> > is the lttng ring buffer design too radically different?
> 
> We tried in the past and never actually got something that we both
> could agree on. Now I believe the user space tools depend on the way
> each one is, so to change one will break the tools that read it.

I understand :-\

thanks,

 - Joel