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    SubjectRe: 2.6.39-rc4+: Kernel leaking memory during FS scanning, regression?
    On Thu, Apr 28, 2011 at 12:32:50AM +0200, Thomas Gleixner wrote:
    > On Wed, 27 Apr 2011, Paul E. McKenney wrote:
    > > On Thu, Apr 28, 2011 at 12:06:11AM +0200, Thomas Gleixner wrote:
    > > > On Wed, 27 Apr 2011, Bruno Prémont wrote:
    > > > > On Wed, 27 April 2011 Bruno Prémont wrote:
    > > > > Voluntary context switches stay constant from the time on SLABs pile up.
    > > > > (which makes sense as it doesn't run get CPU slices anymore)
    > > > >
    > > > > > > Can you please enable CONFIG_SCHED_DEBUG and provide the output of
    > > > > > > /proc/sched_stat when the problem surfaces and a minute after the
    > > > > > > first snapshot?
    > > > >
    > > > > hm, did you mean CONFIG_SCHEDSTAT or /proc/sched_debug?
    > > > >
    > > > > I did use CONFIG_SCHED_DEBUG (and there is no /proc/sched_stat) so I took
    > > > > /proc/sched_debug which exists... (attached, taken about 7min and +1min
    > > > > after SLABs started piling up), though build processes were SIGSTOPped
    > > > > during first minute.
    > > >
    > > > Oops. /proc/sched_debug is the right thing.
    > > >
    > > > > printk wrote (in case its timestamp is useful, more below):
    > > > > [ 518.480103] sched: RT throttling activated
    > > >
    > > > Ok. Aside of the fact that the CPU time accounting is completely hosed
    > > > this is pointing to the root cause of the problem.
    > > >
    > > > kthread_rcu seems to run in circles for whatever reason and the RT
    > > > throttler catches it. After that things go down the drain completely
    > > > as it should get on the CPU again after that 50ms throttling break.
    > >
    > > Ah. This could happen if there was a huge number of callbacks, in
    > > which case blimit would be set very large and kthread_rcu could then
    > > go CPU-bound. And this workload was generating large numbers of
    > > callbacks due to filesystem operations, right?
    > >
    > > So, perhaps I should kick kthread_rcu back to SCHED_NORMAL if blimit
    > > has been set high. Or have some throttling of my own. I must confess
    > > that throttling kthread_rcu for two hours seems a bit harsh. ;-)
    >
    > That's not the intended thing. See below.
    >
    > > If this was just throttling kthread_rcu for a few hundred milliseconds,
    > > or even for a second or two, things would be just fine.
    > >
    > > Left to myself, I will put together a patch that puts callback processing
    > > down to SCHED_NORMAL in the case where there are huge numbers of
    > > callbacks to be processed.
    >
    > Well that's going to paper over the problem at hand possibly. I really
    > don't see why that thing would run for more than 950ms in a row even
    > if there is a large number of callbacks pending.

    True enough, it would probably take millions of callbacks to keep
    rcu_do_batch() busy for 950 milliseconds. Possible, but hopefully
    unlikely.

    Hmmm... If this is happening, I should see it in the debug stuff that
    Sedat sent me. And the biggest change I see in a 15-second interval
    is 50,000 RCU callbacks, which is large, but should not be problematic.
    Even if they all showed up at once, I would hope that they could be
    invoked within a few hundred milliseconds.

    > And then I don't have an explanation for the hosed CPU accounting and
    > why that thing does not get another 950ms RT time when the 50ms
    > throttling break is over.

    Would problems in the CPU accounting result in spurious throttles,
    or are we talking different types of accounting here?

    Thanx, Paul
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