[lttng-dev] Changed scheduling when using lttng

[Mathieu Desnoyers]

* 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.

Thanks,

Mathieu

> 
> /Mats

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