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Date:   Wed, 6 May 2020 17:20:25 +0200
From:   SeongJae Park <sjpark@...zon.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
CC:     SeongJae Park <sjpark@...zon.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        Jakub Kicinski <kuba@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        <sj38.park@...il.com>, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        SeongJae Park <sjpark@...zon.de>, <snu@...zon.com>,
        <amit@...nel.org>, <stable@...r.kernel.org>
Subject: Re: Re: Re: Re: Re: Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change

On Wed, 6 May 2020 07:41:51 -0700 "Paul E. McKenney" <paulmck@...nel.org> wrote:

> On Wed, May 06, 2020 at 02:59:26PM +0200, SeongJae Park wrote:
> > TL; DR: It was not kernel's fault, but the benchmark program.
> > 
> > So, the problem is reproducible using the lebench[1] only.  I carefully read
> > it's code again.
> > 
> > Before running the problem occurred "poll big" sub test, lebench executes
> > "context switch" sub test.  For the test, it sets the cpu affinity[2] and
> > process priority[3] of itself to '0' and '-20', respectively.  However, it
> > doesn't restore the values to original value even after the "context switch" is
> > finished.  For the reason, "select big" sub test also run binded on CPU 0 and
> > has lowest nice value.  Therefore, it can disturb the RCU callback thread for
> > the CPU 0, which processes the deferred deallocations of the sockets, and as a
> > result it triggers the OOM.
> > 
> > We confirmed the problem disappears by offloading the RCU callbacks from the
> > CPU 0 using rcu_nocbs=0 boot parameter or simply restoring the affinity and/or
> > priority.
> > 
> > Someone _might_ still argue that this is kernel problem because the problem
> > didn't occur on the old kernels prior to the Al's patches.  However, setting
> > the affinity and priority was available because the program received the
> > permission.  Therefore, it would be reasonable to blame the system
> > administrators rather than the kernel.
> > 
> > So, please ignore this patchset, apology for making confuse.  If you still has
> > some doubts or need more tests, please let me know.
> > 
> > [1] https://github.com/LinuxPerfStudy/LEBench
> > [2] https://github.com/LinuxPerfStudy/LEBench/blob/master/TEST_DIR/OS_Eval.c#L820
> > [3] https://github.com/LinuxPerfStudy/LEBench/blob/master/TEST_DIR/OS_Eval.c#L822
> 
> Thank you for chasing this down!
> 
> I have had this sort of thing on my list as a potential issue, but given
> that it is now really showing up, it sounds like it is time to bump
> up its priority a bit.  Of course there are limits, so if userspace is
> running at any of the real-time priorities, making sufficient CPU time
> available to RCU's kthreads becomes userspace's responsibility.  But if
> everything is running at SCHED_OTHER (which is this case here, correct?),

Correct.

> then it is reasonable for RCU to do some work to avoid this situation.

That would be also great!

> 
> But still, yes, the immediate job is fixing the benchmark.  ;-)

Totally agreed.

> 
> 							Thanx, Paul
> 
> PS.  Why not just attack all potential issues on my list?  Because I
>      usually learn quite a bit from seeing the problem actually happen.
>      And sometimes other changes in RCU eliminate the potential issue
>      before it has a chance to happen.

Sounds interesting, I will try some of those in my spare time ;)


Thanks,
SeongJae Park

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