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

[Paul E. McKenney]

On Tue, May 05, 2020 at 08:40:07PM +0200, SeongJae Park wrote:
> On Tue, 5 May 2020 11:27:20 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> 
> > On Tue, May 05, 2020 at 07:49:43PM +0200, SeongJae Park wrote:
> > > On Tue, 5 May 2020 10:23:58 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> > > 
> > > > On Tue, May 05, 2020 at 09:25:06AM -0700, 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
> > > > 
> > > > If this is the root cause of the problem, then it will be necessary to
> > > > provide a hint to the hypervisor.  Or, in the near term, avoid loading
> > > > the hypervisor the point that vCPU preemption is so lengthy.
> > > > 
> > > > RCU could also provide some sort of pre-stall-warning notification that
> > > > some of the CPUs aren't passing through quiescent states, which might
> > > > allow the guest OS's userspace to take corrective action.
> > > > 
> > > > But first, what are you doing to either confirm or invalidate the
> > > > hypothesis that this might be due to vCPU preemption?
> > > 
> > > Nothing, I was just guessing.  Sorry if this made you confused.
> > > 
> > > > 
> > > > > >>>>>> 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.
> > > > 
> > > > Is it possible to retest on v5.6?  I have been adding various mechanisms
> > > > to make RCU keep up better with heavy callback overload.
> > > 
> > > I will try soon!
> > > 
> > > > 
> > > > Also, could you please provide the .config?  If either NO_HZ_FULL or
> > > > RCU_NOCB_CPU, please also provide the kernel boot parameters.
> > > 
> > > NO_HZ_FULL is not set, but RCU_NOCB_CPU is y.
> > 
> > OK, this is important information.
> > 
> > > I think I should check whether it's ok to share the full config and boot
> > > parameters.  Please wait this.
> > 
> > I probably don't need the whole thing.  So, if it makes it easier to
> > gain approval...
> > 
> > The main thing I need are CONFIG_PREEMPT and the various Kconfig options
> > having "RCU" in their names.  For example, I have no need for any of the
> > options pertaining to device drivers.  (As far as I know at the moment,
> > anyway!)
> > 
> > For the boot parameters, I am very interested in rcu_nocbs=.  Along with
> > any other boot parameters whose names contain "rcu".
> 
> I guess this would be ok.
> 
> It uses no 'rcu_nocbs=' boot parameter.  

OK, thank you!

For whatever it is worth, if the 'rcu_nocbs=' boot parameters is never
specified, there is no need to build with CONFIG_RCU_NOCB_CPU=y.

> The configs you asked are as below:
> 
>     # CONFIG_PREEMPT is not set
>     
>     #
>     # RCU Subsystem
>     #
>     CONFIG_TREE_RCU=y
>     CONFIG_RCU_EXPERT=y
>     CONFIG_SRCU=y
>     CONFIG_TREE_SRCU=y
>     CONFIG_RCU_STALL_COMMON=y
>     CONFIG_RCU_NEED_SEGCBLIST=y
>     CONFIG_RCU_FANOUT=64
>     CONFIG_RCU_FANOUT_LEAF=16
>     # CONFIG_RCU_FAST_NO_HZ is not set
>     CONFIG_RCU_NOCB_CPU=y
>     # end of RCU Subsystem

Looks pretty standard otherwise.  ;-)

							Thanx, Paul

> > If rcu_nocbs does designate have any CPUs listed, another thing to check
> > is where the rcuo kthreads are permitted to run.  The reason that this
> > is important is that any CPU listed in the rcu_nocbs= boot parameter
> > has its RCU callbacks invoked by one of the rcuo kthreads.  If you have
> > booted with (say) "rcu_nocbs=1,63" and then bound all of the resulting
> > rcuo kthreads to CPU 0, you just tied RCU's hands, making it unable to
> > keep up with any reasonable RCU callback load.
> > 
> > This sort of configuration is permitted, but it is intended for tightly
> > controlled real-time or HPC systems whose configurations and workloads
> > avoid tossing out large numbers of callbacks.  Which might not be the
> > case for your workload.
> > 
> > > > > >> 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.
> > > > 
> > > > As always, "It depends."
> > > > 
> > > > o	If the problem is a too-long RCU reader, RCU is prohibited from
> > > > 	ending the grace period.  The reader duration must be shortened,
> > > > 	and until it is shortened, there is nothing RCU can do.
> > > > 
> > > > o	In some special cases of the above, RCU can and does help, for
> > > > 	example, by enlisting the aid of cond_resched().  So perhaps
> > > > 	there is a long in-kernel loop that needs a cond_resched().
> > > > 
> > > > 	And perhaps RCU can help for some types of vCPU preemption.
> > > > 
> > > > o	As Al suggested offline and as has been discussed in the past,
> > > > 	it would not be hard to cause RCU to burn CPU to attain faster
> > > > 	grace periods during OOM events.  This could be helpful, but only
> > > > 	given that RCU readers are completing in reasonable timeframes.
> > > 
> > > Totally agreed.
> > > 
> > > > > If RCU subsystem can not keep-up, I guess other workloads will also suffer.
> > > > 
> > > > If readers are not excessively long, RCU should be able to keep up.
> > > > (In the absence of misconfigurations, for example, both NO_HZ_FULL and
> > > > then binding all the rcuo kthreads to a single CPU on a 100-CPU system
> > > > or some such.)
> > > > 
> > > > > 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.
> > > > 
> > > > Agreed, let's get more info on what is happening to RCU.
> > > > 
> > > > One approach is to shorten the RCU CPU stall warning timeout
> > > > (rcupdate.rcu_cpu_stall_timeout=10 for 10 seconds).
> > > 
> > > I will also try this and let you know the results.
> > 
> > Sounds good, thank you!
> 
> :)
> 
> 
> Thanks,
> SeongJae Park