[lttng-dev] Changed scheduling when using lttng

Mats Liljegren liljegren.mats2 at gmail.com
Fri Apr 26 10:26:10 EDT 2013

On Fri, Apr 26, 2013 at 3:26 PM, Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
> * Mats Liljegren (liljegren.mats2 at gmail.com) wrote:
>> On Thu, Apr 25, 2013 at 5:12 PM, Mathieu Desnoyers
>> <mathieu.desnoyers at efficios.com> wrote:
>> > Hi Mats,
>> >
>> > The ring buffer uses the standard "timers" in the kernel to flush the
>> > buffers periodically, which prevents your kernel from going into nohz.
>> > Originally, when implemented as a patch on the Linux kernel, the ring
>> > buffer design had hooks in the nohz kernel events to disable this timer
>> > when going to nohz. Now, given LTTng is a kernel module, it cannot
>> > modify the kernel code, and no callback mechanism exists for nohz.
>> >
>> > There are two ways to work around this issue that does not require
>> > modifying the Linux kernel:
>> >
>> > 1) Implement RING_BUFFER_WAKEUP_BY_WRITER within lttng-modules ring
>> >    buffer.
>> >
>> >   it should become used by default if the following is specified at
>> >   channel creation:
>> >
>> >   lttng enable-channel mychan -k --read-timer 0
>> >
>> >   It can be an issue if you want to trace page fault, and instrument
>> >   code sensitive to lock usage (when using WAKEUP_BY_WRITER, the tracer
>> >   is not lock-free anymore). It's the main reason why I have not
>> >   implemented this mode yet: making sure the tracer never breaks the
>> >   kernel in this mode is trickier.
>> >
>> > 2) use deferrable timers. It's a hack, but it should allow our timers to
>> >    be inhibited when the cpus go in nohz.
>> >
>> > Sorry, low-impact on nohz has not really been on our sponsor's priority
>> > lists so far.
>> >
>> > Thanks,
>> >
>> > Mathieu
>> I tried number 1 using --read-timer 0, but "lttng stop" hanged at
>> "Waiting for data availability", producing lots of dots...
> As I said, we'd need to implement RING_BUFFER_WAKEUP_BY_WRITER when
> read-timer is set to 0. It's not implemented currently.
>> Would it be possible to let some other (not using nohz mode) CPU to
>> flush the buffers?
> I guess that would be option 3) :
> Another option would be to let a single thread in the consumer handle
> the read-timer for all streams of the channel, like we do for UST.

Ehm, well, you did say something about implement... Sorry for missing that.

I guess now the question is which option that gives best
characteristics for least amount of work... Without knowing the design
of lttng-module, I'd believe that simply having the timer on another
CPU should be a good candidate. Is there anything to watch out for
with this solution?

Are there any documents describing lttng-module design, or is it "join
the force, use the source"? I've seen some high-level description
showing how tools/libs/modules fit together, but I haven't found
anything that describes how lttng-modules is designed.


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