Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change

[Paul E. McKenney]

On Tue, May 05, 2020 at 09:37:42AM -0700, Eric Dumazet 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@xxxxxxxxxx> wrote:
> >>>
> >>>> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
> >>>>>
> >>>>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> >>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 5/5/20 8:07 AM, SeongJae Park wrote:
> >>>>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >>>>>>>
> >>>>>>
> >>>>>>>> Why do we have 10,000,000 objects around ? Could this be because of
> >>>>>>>> some RCU problem ?
> >>>>>>>
> >>>>>>> Mainly because of a long RCU grace period, as you guess.  I have no idea how
> >>>>>>> the grace period became so long in this case.
> >>>>>>>
> >>>>>>> As my test machine was a virtual machine instance, I guess RCU readers
> >>>>>>> preemption[1] like problem might affected this.
> >>>>>>>
> >>>>>>> [1] https://www.usenix.org/system/files/conference/atc17/atc17-prasad.pdf
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Once Al patches reverted, do you have 10,000,000 sock_alloc around ?
> >>>>>>>
> >>>>>>> Yes, both the old kernel that prior to Al's patches and the recent kernel
> >>>>>>> reverting the Al's patches didn't reproduce the problem.
> >>>>>>>
> >>>>>>
> >>>>>> I repeat my question : Do you have 10,000,000 (smaller) objects kept in slab caches ?
> >>>>>>
> >>>>>> TCP sockets use the (very complex, error prone) SLAB_TYPESAFE_BY_RCU, but not the struct socket_wq
> >>>>>> object that was allocated in sock_alloc_inode() before Al patches.
> >>>>>>
> >>>>>> These objects should be visible in kmalloc-64 kmem cache.
> >>>>>
> >>>>> Not exactly the 10,000,000, as it is only the possible highest number, but I
> >>>>> was able to observe clear exponential increase of the number of the objects
> >>>>> using slabtop.  Before the start of the problematic workload, the number of
> >>>>> objects of 'kmalloc-64' was 5760, but I was able to observe the number increase
> >>>>> to 1,136,576.
> >>>>>
> >>>>>           OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
> >>>>> before:   5760   5088  88%    0.06K     90       64       360K kmalloc-64
> >>>>> after:  1136576 1136576 100%    0.06K  17759       64     71036K kmalloc-64
> >>>>>
> >>>>
> >>>> Great, thanks.
> >>>>
> >>>> How recent is the kernel you are running for your experiment ?
> >>>
> >>> It's based on 5.4.35.
> >>>
> >>>>
> >>>> Let's make sure the bug is not in RCU.
> >>>
> >>> One thing I can currently say is that the grace period passes at last.  I
> >>> modified the benchmark to repeat not 10,000 times but only 5,000 times to run
> >>> the test without OOM but easily observable memory pressure.  As soon as the
> >>> benchmark finishes, the memory were freed.
> >>>
> >>> If you need more tests, please let me know.
> >>>
> >>
> >> 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

Worth a try!  If that helps significantly, I have some ideas for updating
that heuristic, such as checking for sudden increases in the number of
pending callbacks.

But I would really also like to know whether there are long readers and
whether v5.6 fares better.

							Thanx, Paul