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Date:   Tue, 5 May 2020 11:17:07 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     SeongJae Park <sjpark@...zon.com>
Cc:     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: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle
 change

On Tue, May 05, 2020 at 07:56:05PM +0200, SeongJae Park wrote:
> On Tue, 5 May 2020 10:30:36 -0700 "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > On Tue, May 05, 2020 at 07:05:53PM +0200, SeongJae Park wrote:
> > > On Tue, 5 May 2020 09:37:42 -0700 Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > 
> > > > 
> > > > 
> > > > On 5/5/20 9:31 AM, Eric Dumazet wrote:
> > > > > 
> > > > > 
> > > > > On 5/5/20 9:25 AM, Eric Dumazet wrote:
> > > > >>
> > > > >>
> > > > >> On 5/5/20 9:13 AM, SeongJae Park wrote:
> > > > >>> On Tue, 5 May 2020 09:00:44 -0700 Eric Dumazet <edumazet@...gle.com> wrote:
> > > > >>>
> > > > >>>> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjpark@...zon.com> wrote:
> > > > >>>>>
> > > > >>>>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > > >>>>>
> > > > >>>>>>
> > > > >>>>>>
> > > > >>>>>> On 5/5/20 8:07 AM, SeongJae Park wrote:
> > > > >>>>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@...gle.com> wrote:
> > > > >>>>>>>
> > > > >>>>>>
> > > [...]
> > > > >>
> > > > >> I would ask Paul opinion on this issue, because we have many objects
> > > > >> being freed after RCU grace periods.
> > > > >>
> > > > >> If RCU subsystem can not keep-up, I guess other workloads will also suffer.
> > > > >>
> > > > >> Sure, we can revert patches there and there trying to work around the issue,
> > > > >> but for objects allocated from process context, we should not have these problems.
> > > > >>
> > > > > 
> > > > > I wonder if simply adjusting rcu_divisor to 6 or 5 would help 
> > > > > 
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index d9a49cd6065a20936edbda1b334136ab597cde52..fde833bac0f9f81e8536211b4dad6e7575c1219a 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -427,7 +427,7 @@ module_param(qovld, long, 0444);
> > > > >  static ulong jiffies_till_first_fqs = ULONG_MAX;
> > > > >  static ulong jiffies_till_next_fqs = ULONG_MAX;
> > > > >  static bool rcu_kick_kthreads;
> > > > > -static int rcu_divisor = 7;
> > > > > +static int rcu_divisor = 6;
> > > > >  module_param(rcu_divisor, int, 0644);
> > > > >  
> > > > >  /* Force an exit from rcu_do_batch() after 3 milliseconds. */
> > > > > 
> > > > 
> > > > To be clear, you can adjust the value without building a new kernel.
> > > > 
> > > > echo 6 >/sys/module/rcutree/parameters/rcu_divisor
> > > 
> > > I tried value 6, 5, and 4, but none of those removed the problem.
> > 
> > Thank you for checking this!
> > 
> > Was your earlier discussion on long RCU readers speculation, or do you
> > have measurements?
> 
> It was just a guess without any measurement or dedicated investigation.

OK, another thing to check is the duration of the low-memory episode.
Does this duration exceed the RCU CPU stall warning time?  (21 seconds
in mainline, 60 in many distros, but check rcupdate.rcu_cpu_stall_timeout
to be sure.)

Also, any chance of a .config?  Or at least the RCU portions?  I am
guessing CONFIG_PREEMPT=n, for example.

							Thanx, Paul

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