Message-ID: <CAOzhvcNiaFM9pAEURdoZSUo8nVfBMMA-+Fwd6QBWr2pk_j48cw@mail.gmail.com>
Date:   Wed, 20 Oct 2021 16:05:59 -0400
From:   donghai qiao <donghai.w.qiao@...il.com>
To:     paulmck@...nel.org
Cc:     Boqun Feng <boqun.feng@...il.com>, rcu@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: RCU: rcu stall issues and an approach to the fix

On Wed, Oct 20, 2021 at 2:37 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Wed, Oct 20, 2021 at 01:48:15PM -0400, donghai qiao wrote:
> > On Mon, Oct 18, 2021 at 7:46 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > > On Mon, Oct 18, 2021 at 05:18:40PM -0400, donghai qiao wrote:
> > > > I just want to follow up this discussion. First off, the latest issue
> > > > I mentioned in the email of Oct 4th which
> > > > exhibited a symptom of networking appeared to be a problem in
> > > > qrwlock.c. Particularly the problem is
> > > > caused by the 'if' statement in the function queued_read_lock_slowpath() below :
> > > >
> > > > void queued_read_lock_slowpath(struct qrwlock *lock)
> > > > {
> > > >         /*
> > > >          * Readers come here when they cannot get the lock without waiting
> > > >          */
> > > >         if (unlikely(in_interrupt())) {
> > > >                 /*
> > > >                  * Readers in interrupt context will get the lock immediately
> > > >                  * if the writer is just waiting (not holding the lock yet),
> > > >                  * so spin with ACQUIRE semantics until the lock is available
> > > >                  * without waiting in the queue.
> > > >                  */
> > > >                 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> > > >                 return;
> > > >         }
> > > >         ...
> > > > }
> > > >
> > > > That 'if' statement said, if we are in an interrupt context and we are
> > > > a reader, then
> > > > we will be allowed to enter the lock as a reader no matter if there
> > > > are writers waiting
> > > > for it or not. So, in the circumstance when the network packets steadily come in
> > > > and the intervals are relatively small enough, then the writers will
> > > > have no chance to
> > > > acquire the lock. This should be the root cause for that case.
> > > >
> > > > I have verified it by removing the 'if' and rerun the test multiple
> > > > times.
> > >
> > > That could do it!
> > >
> > > Would it make sense to keep the current check, but to also check if a
> > > writer had been waiting for more than (say) 100ms?  The reason that I
> > > ask is that I believe that this "if" statement is there for a reason.
> >
> > The day before I also got this to Waiman Long who initially made these
> > changes in
> > the qrwlock.c file. Turns out, the 'if' block was introduced to
> > resolve the particular
> > requirement of tasklist_lock reentering as reader.  He said he will
> > perhaps come up
> > with another code change to take care of this new write lock
> > starvation issue. The
> > idea is to only allow the tasklist_lock clients to acquire the read
> > lock through the 'if'
> > statement,  others are not.
> >
> > This sounds like a temporary solution if we cannot think of other
> > alternative ways
> > to fix the tasklist_lock issue. The principle here is that we should
> > not make the
> > locking primitives more special just in favor of a particular usage or scenario.
>
> When principles meet practice, results can vary.  Still, it would be
> better to have a less troublesome optimization.
>

This is a philosophical debate. Let's put it aside.

> > > >        The same
> > > > symptom hasn't been reproduced.  As far as rcu stall is concerned as a
> > > > broader range
> > > > of problems,  this is absolutely not the only root cause I have seen.
> > > > Actually too many
> > > > things can delay context switching.  Do you have a long term plan to
> > > > fix this issue,
> > > > or just want to treat it case by case?
> > >
> > > If you are running a CONFIG_PREEMPT=n kernel, then the plan has been to
> > > leverage the calls to cond_resched().  If the grace period is old enough,
> > > cond_resched() will supply a quiescent state.
> >
> > So far, all types of rcu stall I am aware of are originated to the
> > CONFIG_PREEMPT=n
> > kernel. Isn't it impossible to let rcu not rely on context switch ?
> > As we know too many
> > things can delay context switch, so it is not a quite reliable
> > mechanism if timing and
> > performance are crucial.
>
> Yes, you could build with CONFIG_PREEMPT=y and RCU would not always
> need to wait for an actual context switch.  But there can be
> performance issues for some workloads.
>
I can give this config (CONFIG_PREEMPT=y) a try when I have time.

> But please note that cond_resched() is not necessarily a context switch.
>
> Besides, for a great many workloads, delaying a context switch for
> very long is a first-class bug anyway.  For example, many internet data
> centers are said to have sub-second response-time requirements, and such
> requirements cannot be met if context switches are delayed too long.
>
Agreed.

But on the other hand, if rcu relies on that,  the situation could be
even worse.
Simply put, when a gp cannot end soon, some rcu write-side will be delayed,
and the callbacks on the rcu-stalled CPU will be delayed. Thus in the case of
lack of free memory, this situation could form a deadlock.

> > > In a CONFIG_PREEMPT=y kernel, when the grace period is old enough,
> > > RCU forces a schedule on the holdout CPU.  As long as the CPU is not
> > > eternally non-preemptible (for example, eternally in an interrupt
> > > handler), the grace period will end.
> >
> > Among the rcu stall instances I have seen so far, quite a lot of them occurred
> > on the CPUs which were running in the interrupt context or spinning on spinlocks
> > with interrupt disabled. In these scenarios, forced schedules will be
> > delayed until
> > these activities end.
>
> But running for several seconds in interrupt context is not at all good.
> As is spinning on a spinlock for several seconds.  These are performance
> bugs in and of themselves.

Agreed.

>
> More on this later in this email...
>
> > > But beyond a certain point, case-by-case analysis and handling is
> > > required.
> > >
> > > > Secondly, back to the following code I brought up that day. Actually
> > > > it is not as simple
> > > > as spinlock.
> > > >
> > > >       rcu_read_lock();
> > > >       ip_protocol_deliver_rcu(net, skb, ip_hdr(skb)->protocol);
> > > >       rcu_read_unlock();
> > > >
> > > > Are you all aware of all the potential functions that
> > > > ip_protocol_deliver_rcu will call?
> > > > As I can see, there is a code path from ip_protocol_deliver_rcu to
> > > > kmem_cache_alloc
> > > > which will end up a call to cond_resched().
> > >
> > > Can't say that I am familiar with everything that ip_protocol_deliver_rcu().
> > > There are some tens of millions of lines of code in the kernel, and I have
> > > but one brain.  ;-)
> > >
> > > And this cond_resched() should set things straight for a CONFIG_PREEMPT=n
> > > kernel.  Except that there should not be a call to cond_resched() within
> > > an RCU read-side critical section.
> >
> > with that 3 line snippet from the networking, a call to cond_resched() would
> > happen within the read-side critical section when the level of variable memory
> > is very low.
>
> That is a bug.  If you build your kernel with CONFIG_PROVE_LOCKING=y,
> it will complain about a cond_resched() in an RCU read-side critical
> section.  But, as you say, perhaps only with the level of variable memory
> is very low.

There is a typo in my previous email. I meant available (or free).
Sorry for that.

>
> Please do not invoke cond_resched() within an RCU read-side critical
> section.  Doing so can result in random memory corruption.
>
> > > Does the code momentarily exit that
> > > critical section via something like rcu_read_unlock(); cond_resched();
> > > rcu_read_lock()?
> >
> > As far as I can see, cond_resched would be called between a pair of
> > rcu_read_lock and rcu_read_unlock.
>
> Again, this is a bug.  The usual fix is the GFP_ thing I noted below.
>
> > > Or does something prevent the code from getting there
> > > while in an RCU read-side critical section?  (The usual trick here is
> > > to have different GFP_ flags depending on the context.)
> >
> > Once we invoke kmem_cache_alloc or its variants, we cannot really
> > predict where we will go and how long this whole process is going to
> > take in this very large area from kmem to the virtual memory subsystem.
> > There is a flag __GFP_NOFAIL that determines whether or not cond_resched
> > should be called before retry, but this flag should be used from page level,
> > not from the kmem consumer level.  So I think there is little we can do
> > to avoid the resched.
>
> If you are invoking the allocator within an RCU read-side critical
> section, you should be using GFP_ATOMIC.  Except that doing this has
> many negative consequences, so it is better to allocate outside of
> the RCU read-side critical section.
>
> The same rules apply when allocating while holding a spinlock, so
> this is not just RCU placing restrictions on you.  ;-)

yep, absolutely.

>
> > > >                                              Because the operations in memory
> > > > allocation are too complicated, we cannot alway expect a prompt return
> > > > with success.
> > > > When the system is running out of memory, then rcu cannot close the
> > > > current gp, then
> > > > great number of callbacks will be delayed and the freeing of the
> > > > memory they held
> > > > will be delayed as well. This sounds like a deadlock in the resource flow.
> > >
> > > That would of course be bad.  Though I am not familiar with all of the
> > > details of how the networking guys handle out-of-memory conditions.
> > >
> > > The usual advice would be to fail the request, but that does not appear
> > > to be an easy option for ip_protocol_deliver_rcu().  At this point, I
> > > must defer to the networking folks.
> >
> > Thanks for the advice.
>
> Another question...  Why the endless interrupts?  Or is it just one
> very long interrupt?  Last I knew (admittedly a very long time ago),
> the high-rate networking drivers used things like NAPI in order to avoid
> this very problem.

These should be long enough interrupts. The symptom in the networking
as the previous email said is one of them.  In that case, due to  rwlock
in favor of the readers in the interrupt context, the writer side would be
blocked as long as the readers keep coming.

>
> Or is this some sort of special case where you are trying to do something
> special, for example, to achieve extremely low communications latencies?

No, nothing special I am trying to do.

>
> If this is a deliberate design, and if it is endless interrupts instead
> of one big long one, and if you are deliberately interrupt-storming
> a particular CPU, another approach is to build the kernel with
> CONFIG_NO_HZ_FULL=y, and boot with nohz_full=n, where "n" is the number of
> the CPU that is to be interrupt-stormed.  If you are interrupt storming
> multiple CPUs, you can specify them, for example, nohz_full=1-5,13 to
> specify CPUs 1, 2, 3, 4, 5, and 13.  In recent kernels, "N" stands for
> the CPU with the largest CPU number.

I did this before, and I saw rcu stall as well with this kinda config.

>
> Then read Documentation/admin-guide/kernel-per-CPU-kthreads.rst, which is
> probably a bit outdated, but a good place to start.  Follow its guidelines
> (and, as needed, come up with additional ones) to ensure that CPU "n"
> is not doing anything.  If you do come up with additional guidelines,
> please submit a patch to kernel-per-CPU-kthreads.rst so that others can
> also benefit, as you are benefiting from those before you.

Thanks for the suggestion.
>
> Create a CPU-bound usermode application (a "while (1) continue;" loop or
> similar), and run that application on CPU "n".  Then start up whatever
> it is that interrupt-storms CPU "n".
>
> Every time CPU "n" returns from interrupt, RCU will see a quiescent state,
> which will prevent the interrupt storm from delaying RCU grace periods.
>
> On the other hand, if this is one big long interrupt, you need to make
> that interrupt end every so often.  Or move some of the work out of
> interrupt context, perhaps even to usermode.
>
> Much depends on exactly what you are trying to achieve.

The things that can affect rcu stall are too many. So let's deal with
it case by case
before there is a permanent solution.

Thanks
Donghai




>
>                                                         Thanx, Paul
>
> > Donghai
> > >
> > >                                                         Thanx, Paul
> > >
> > > > Thanks
> > > > Donghai
> > > >
> > > >
> > > > On Tue, Oct 5, 2021 at 8:25 PM donghai qiao <donghai.w.qiao@...il.com> wrote:
> > > > >
> > > > > On Tue, Oct 5, 2021 at 12:39 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > >
> > > > > > On Tue, Oct 05, 2021 at 12:10:25PM -0400, donghai qiao wrote:
> > > > > > > On Mon, Oct 4, 2021 at 8:59 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > > > >
> > > > > > > > On Mon, Oct 04, 2021 at 05:22:52PM -0400, donghai qiao wrote:
> > > > > > > > > Hello Paul,
> > > > > > > > > Sorry it has been long..
> > > > > > > >
> > > > > > > > On this problem, your schedule is my schedule.  At least as long as your
> > > > > > > > are not expecting instantaneous response.  ;-)
> > > > > > > >
> > > > > > > > > > > Because I am dealing with this issue in multiple kernel versions, sometimes
> > > > > > > > > > > the configurations in these kernels may different. Initially the
> > > > > > > > > > > problem I described
> > > > > > > > > > > originated to rhel-8 on which the problem occurs more often and is a bit easier
> > > > > > > > > > > to reproduce than others.
> > > > > > > > > >
> > > > > > > > > > Understood, that does make things more difficult.
> > > > > > > > > >
> > > > > > > > > > > Regarding these dynticks* parameters, I collected the data for CPU 0 as below :
> > > > > > > > > > >    - dynticks = 0x6eab02    which indicated the CPU was not in eqs.
> > > > > > > > > > >    - dynticks_nesting = 1    which is in its initial state, so it said
> > > > > > > > > > > it was not in eqs either.
> > > > > > > > > > >    - dynticks_nmi_nesting = 4000000000000004    which meant that this
> > > > > > > > > > > CPU had been
> > > > > > > > > > >      interrupted when it was in the middle of the first interrupt.
> > > > > > > > > > > And this is true: the first
> > > > > > > > > > >      interrupt was the sched_timer interrupt, and the second was a NMI
> > > > > > > > > > > when another
> > > > > > > > > > >     CPU detected the RCU stall on CPU 0.  So it looks all identical.
> > > > > > > > > > > If the kernel missed
> > > > > > > > > > >     a rcu_user_enter or rcu_user_exit, would these items remain
> > > > > > > > > > > identical ?  But I'll
> > > > > > > > > > >     investigate that possibility seriously as you pointed out.
> > > > > > > > > >
> > > > > > > > > > So is the initial state non-eqs because it was interrupted from kernel
> > > > > > > > > > mode?  Or because a missing rcu_user_enter() left ->dynticks_nesting
> > > > > > > > > > incorrectly equal to the value of 1?  Or something else?
> > > > > > > > >
> > > > > > > > > As far as the original problem is concerned, the user thread was interrupted by
> > > > > > > > > the timer, so the CPU was not working in the nohz mode. But I saw the similar
> > > > > > > > > problems on CPUs working in nohz mode with different configurations.
> > > > > > > >
> > > > > > > > OK.
> > > > > > > >
> > > > > > > > > > > > There were some issues of this sort around the v5.8 timeframe.  Might
> > > > > > > > > > > > there be another patch that needs to be backported?  Or a patch that
> > > > > > > > > > > > was backported, but should not have been?
> > > > > > > > > > >
> > > > > > > > > > > Good to know that clue. I'll take a look into the log history.
> > > > > > > > > > >
> > > > > > > > > > > > Is it possible to bisect this?
> > > > > > > > > > > >
> > > > > > > > > > > > Or, again, to run with CONFIG_RCU_EQS_DEBUG=y?
> > > > > > > > > > >
> > > > > > > > > > > I am building the latest 5.14 kernel with this config and give it a try when the
> > > > > > > > > > > machine is set up, see how much it can help.
> > > > > > > > > >
> > > > > > > > > > Very good, as that will help determine whether or not the problem is
> > > > > > > > > > due to backporting issues.
> > > > > > > > >
> > > > > > > > > I enabled CONFIG_RCU_EQS_DEBUG=y as you suggested and
> > > > > > > > > tried it for both the latest rhel8 and a later upstream version 5.15.0-r1,
> > > > > > > > > turns out no new warning messages related to this came out. So,
> > > > > > > > > rcu_user_enter/rcu_user_exit() should be paired right.
> > > > > > > >
> > > > > > > > OK, good.
> > > > > > > >
> > > > > > > > > > > > Either way, what should happen is that dyntick_save_progress_counter() or
> > > > > > > > > > > > rcu_implicit_dynticks_qs() should see the rdp->dynticks field indicating
> > > > > > > > > > > > nohz_full user execution, and then the quiescent state will be supplied
> > > > > > > > > > > > on behalf of that CPU.
> > > > > > > > > > >
> > > > > > > > > > > Agreed. But the counter rdp->dynticks of the CPU can only be updated
> > > > > > > > > > > by rcu_dynticks_eqs_enter() or rcu_dynticks_exit() when rcu_eqs_enter()
> > > > > > > > > > > or rcu_eqs_exit() is called, which in turn depends on the context switch.
> > > > > > > > > > > So, when the context switch never happens, the counter rdp->dynticks
> > > > > > > > > > > never advances. That's the thing I try to fix here.
> > > > > > > > > >
> > > > > > > > > > First, understand the problem.  Otherwise, your fix is not so likely
> > > > > > > > > > to actually fix anything.  ;-)
> > > > > > > > > >
> > > > > > > > > > If kernel mode was interrupted, there is probably a missing cond_resched().
> > > > > > > > > > But in sufficiently old kernels, cond_resched() doesn't do anything for
> > > > > > > > > > RCU unless a context switch actually happened.  In some of those kernels,
> > > > > > > > > > you can use cond_resched_rcu_qs() instead to get RCU's attention.  In
> > > > > > > > > > really old kernels, life is hard and you will need to do some backporting.
> > > > > > > > > > Or move to newer kernels.
> > > > > > > > > >
> > > > > > > > > > In short, if an in-kernel code path runs for long enough without hitting
> > > > > > > > > > a cond_resched() or similar, that is a bug.  The RCU CPU stall warning
> > > > > > > > > > that you will get is your diagnostic.
> > > > > > > > >
> > > > > > > > > Probably this is the case. With the test for 5.15.0-r1, I have seen different
> > > > > > > > > scenarios, among them the most frequent ones were caused by the networking
> > > > > > > > > in which a bunch of networking threads were spinning on the same rwlock.
> > > > > > > > >
> > > > > > > > > For instance in one of them, the ticks_this_gp of a rcu_data could go as
> > > > > > > > > large as 12166 (ticks) which is 12+ seconds. The thread on this cpu was
> > > > > > > > > doing networking work and finally it was spinning as a writer on a rwlock
> > > > > > > > > which had been locked by 16 readers.  By the way, there were 70 this
> > > > > > > > > kinds of writers were blocked on the same rwlock.
> > > > > > > >
> > > > > > > > OK, a lock-contention problem.  The networking folks have fixed a
> > > > > > > > very large number of these over the years, though, so I wonder what is
> > > > > > > > special about this one so that it is just now showing up.  I have added
> > > > > > > > a networking list on CC for their thoughts.
> > > > > > >
> > > > > > > Thanks for pulling the networking in. If they need the coredump, I can
> > > > > > > forward it to them.  It's definitely worth analyzing it as this contention
> > > > > > > might be a performance issue.  Or we can discuss this further in this
> > > > > > > email thread if they are fine, or we can discuss it over with a separate
> > > > > > > email thread with netdev@ only.
> > > > > > >
> > > > > > > So back to my original problem, this might be one of the possibilities that
> > > > > > > led to RCU stall panic.  Just imagining this type of contention might have
> > > > > > > occurred and lasted long enough. When it finally came to the end, the
> > > > > > > timer interrupt occurred, therefore rcu_sched_clock_irq detected the RCU
> > > > > > > stall on the CPU and panic.
> > > > > > >
> > > > > > > So definitely we need to understand these networking activities here as
> > > > > > > to why the readers could hold the rwlock too long.
> > > > > >
> > > > > > I strongly suggest that you also continue to do your own analysis on this.
> > > > > > So please see below.
> > > > >
> > > > > This is just a brief of my analysis and the stack info below is not enough
> > > > > for other people to figure out anything useful. I meant if they are really
> > > > > interested, I can upload the core file. I think this is fair.
> > > > >
> > > > > >
> > > > > > > > > When examining the readers of the lock, except the following code,
> > > > > > > > > don't see any other obvious problems: e.g
> > > > > > > > >  #5 [ffffad3987254df8] __sock_queue_rcv_skb at ffffffffa49cd2ee
> > > > > > > > >  #6 [ffffad3987254e18] raw_rcv at ffffffffa4ac75c8
> > > > > > > > >  #7 [ffffad3987254e38] raw_local_deliver at ffffffffa4ac7819
> > > > > > > > >  #8 [ffffad3987254e88] ip_protocol_deliver_rcu at ffffffffa4a8dea4
> > > > > > > > >  #9 [ffffad3987254ea8] ip_local_deliver_finish at ffffffffa4a8e074
> > > > > > > > > #10 [ffffad3987254eb0] __netif_receive_skb_one_core at ffffffffa49f3057
> > > > > > > > > #11 [ffffad3987254ed0] process_backlog at ffffffffa49f3278
> > > > > > > > > #12 [ffffad3987254f08] __napi_poll at ffffffffa49f2aba
> > > > > > > > > #13 [ffffad3987254f30] net_rx_action at ffffffffa49f2f33
> > > > > > > > > #14 [ffffad3987254fa0] __softirqentry_text_start at ffffffffa50000d0
> > > > > > > > > #15 [ffffad3987254ff0] do_softirq at ffffffffa40e12f6
> > > > > > > > >
> > > > > > > > > In the function ip_local_deliver_finish() of this stack, a lot of the work needs
> > > > > > > > > to be done with ip_protocol_deliver_rcu(). But this function is invoked from
> > > > > > > > > a rcu reader side section.
> > > > > > > > >
> > > > > > > > > static int ip_local_deliver_finish(struct net *net, struct sock *sk,
> > > > > > > > > struct sk_buff *skb)
> > > > > > > > > {
> > > > > > > > >         __skb_pull(skb, skb_network_header_len(skb));
> > > > > > > > >
> > > > > > > > >         rcu_read_lock();
> > > > > > > > >         ip_protocol_deliver_rcu(net, skb, ip_hdr(skb)->protocol);
> > > > > > > > >         rcu_read_unlock();
> > > > > > > > >
> > > > > > > > >         return 0;
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > Actually there are multiple chances that this code path can hit
> > > > > > > > > spinning locks starting from ip_protocol_deliver_rcu(). This kind
> > > > > > > > > usage looks not quite right. But I'd like to know your opinion on this first ?
> > > > > > > >
> > > > > > > > It is perfectly legal to acquire spinlocks in RCU read-side critical
> > > > > > > > sections.  In fact, this is one of the few ways to safely acquire a
> > > > > > > > per-object lock while still maintaining good performance and
> > > > > > > > scalability.
> > > > > > >
> > > > > > > Sure, understand. But the RCU related docs said that anything causing
> > > > > > > the reader side to block must be avoided.
> > > > > >
> > > > > > True.  But this is the Linux kernel, where "block" means something
> > > > > > like "invoke schedule()" or "sleep" instead of the academic-style
> > > > > > non-blocking-synchronization definition.  So it is perfectly legal to
> > > > > > acquire spinlocks within RCU read-side critical sections.
> > > > > >
> > > > > > And before you complain that practitioners are not following the academic
> > > > > > definitions, please keep in mind that our definitions were here first.  ;-)
> > > > > >
> > > > > > > > My guess is that the thing to track down is the cause of the high contention
> > > > > > > > on that reader-writer spinlock.  Missed patches, misconfiguration, etc.
> > > > > > >
> > > > > > > Actually, the test was against a recent upstream 5.15.0-r1  But I can try
> > > > > > > the latest r4.  Regarding the network configure, I believe I didn't do anything
> > > > > > > special, just use the default.
> > > > > >
> > > > > > Does this occur on older mainline kernels?  If not, I strongly suggest
> > > > > > bisecting, as this often quickly and easily finds the problem.
> > > > >
> > > > > Actually It does. But let's focus on the latest upstream and the latest rhel8.
> > > > > This way, we will not worry about missing the needed rcu patches.
> > > > > However, in rhel8, the kernel stack running on the rcu-stalled CPU is not
> > > > > networking related, which I am still working on.  So, there might be
> > > > > multiple root causes.
> > > > >
> > > > > > Bisection can also help you find the patch to be backported if a later
> > > > > > release fixes the bug, though things like gitk can also be helpful.
> > > > >
> > > > > Unfortunately, this is reproducible on the latest bit.
> > > > >
> > > > > Thanks
> > > > > Donghai
> > > > > >
> > > > > >                                                         Thanx, Paul

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