Message-ID: <877dkygeca.fsf@toke.dk>
Date:   Mon, 19 Apr 2021 23:21:41 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     paulmck@...nel.org
Cc:     Martin KaFai Lau <kafai@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Hangbin Liu <liuhangbin@...il.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Jiri Benc <jbenc@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel 
        <bjorn.topel@...il.com>
Subject: Re: [PATCHv7 bpf-next 1/4] bpf: run devmap xdp_prog on flush
 instead of bulk enqueue

"Paul E. McKenney" <paulmck@...nel.org> writes:

> On Mon, Apr 19, 2021 at 08:12:27PM +0200, Toke Høiland-Jørgensen wrote:
>> "Paul E. McKenney" <paulmck@...nel.org> writes:
>> 
>> > On Sat, Apr 17, 2021 at 02:27:19PM +0200, Toke Høiland-Jørgensen wrote:
>> >> "Paul E. McKenney" <paulmck@...nel.org> writes:
>> >> 
>> >> > On Fri, Apr 16, 2021 at 11:22:52AM -0700, Martin KaFai Lau wrote:
>> >> >> On Fri, Apr 16, 2021 at 03:45:23PM +0200, Jesper Dangaard Brouer wrote:
>> >> >> > On Thu, 15 Apr 2021 17:39:13 -0700
>> >> >> > Martin KaFai Lau <kafai@...com> wrote:
>> >> >> > 
>> >> >> > > On Thu, Apr 15, 2021 at 10:29:40PM +0200, Toke Høiland-Jørgensen wrote:
>> >> >> > > > Jesper Dangaard Brouer <brouer@...hat.com> writes:
>> >> >> > > >   
>> >> >> > > > > On Thu, 15 Apr 2021 10:35:51 -0700
>> >> >> > > > > Martin KaFai Lau <kafai@...com> wrote:
>> >> >> > > > >  
>> >> >> > > > >> On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke Høiland-Jørgensen wrote:  
>> >> >> > > > >> > Hangbin Liu <liuhangbin@...il.com> writes:
>> >> >> > > > >> >     
>> >> >> > > > >> > > On Wed, Apr 14, 2021 at 05:17:11PM -0700, Martin KaFai Lau wrote:    
>> >> >> > > > >> > >> >  static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
>> >> >> > > > >> > >> >  {
>> >> >> > > > >> > >> >  	struct net_device *dev = bq->dev;
>> >> >> > > > >> > >> > -	int sent = 0, err = 0;
>> >> >> > > > >> > >> > +	int sent = 0, drops = 0, err = 0;
>> >> >> > > > >> > >> > +	unsigned int cnt = bq->count;
>> >> >> > > > >> > >> > +	int to_send = cnt;
>> >> >> > > > >> > >> >  	int i;
>> >> >> > > > >> > >> >  
>> >> >> > > > >> > >> > -	if (unlikely(!bq->count))
>> >> >> > > > >> > >> > +	if (unlikely(!cnt))
>> >> >> > > > >> > >> >  		return;
>> >> >> > > > >> > >> >  
>> >> >> > > > >> > >> > -	for (i = 0; i < bq->count; i++) {
>> >> >> > > > >> > >> > +	for (i = 0; i < cnt; i++) {
>> >> >> > > > >> > >> >  		struct xdp_frame *xdpf = bq->q[i];
>> >> >> > > > >> > >> >  
>> >> >> > > > >> > >> >  		prefetch(xdpf);
>> >> >> > > > >> > >> >  	}
>> >> >> > > > >> > >> >  
>> >> >> > > > >> > >> > -	sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
>> >> >> > > > >> > >> > +	if (bq->xdp_prog) {    
>> >> >> > > > >> > >> bq->xdp_prog is used here
>> >> >> > > > >> > >>     
>> >> >> > > > >> > >> > +		to_send = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
>> >> >> > > > >> > >> > +		if (!to_send)
>> >> >> > > > >> > >> > +			goto out;
>> >> >> > > > >> > >> > +
>> >> >> > > > >> > >> > +		drops = cnt - to_send;
>> >> >> > > > >> > >> > +	}
>> >> >> > > > >> > >> > +    
>> >> >> > > > >> > >> 
>> >> >> > > > >> > >> [ ... ]
>> >> >> > > > >> > >>     
>> >> >> > > > >> > >> >  static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
>> >> >> > > > >> > >> > -		       struct net_device *dev_rx)
>> >> >> > > > >> > >> > +		       struct net_device *dev_rx, struct bpf_prog *xdp_prog)
>> >> >> > > > >> > >> >  {
>> >> >> > > > >> > >> >  	struct list_head *flush_list = this_cpu_ptr(&dev_flush_list);
>> >> >> > > > >> > >> >  	struct xdp_dev_bulk_queue *bq = this_cpu_ptr(dev->xdp_bulkq);
>> >> >> > > > >> > >> > @@ -412,18 +466,22 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
>> >> >> > > > >> > >> >  	/* Ingress dev_rx will be the same for all xdp_frame's in
>> >> >> > > > >> > >> >  	 * bulk_queue, because bq stored per-CPU and must be flushed
>> >> >> > > > >> > >> >  	 * from net_device drivers NAPI func end.
>> >> >> > > > >> > >> > +	 *
>> >> >> > > > >> > >> > +	 * Do the same with xdp_prog and flush_list since these fields
>> >> >> > > > >> > >> > +	 * are only ever modified together.
>> >> >> > > > >> > >> >  	 */
>> >> >> > > > >> > >> > -	if (!bq->dev_rx)
>> >> >> > > > >> > >> > +	if (!bq->dev_rx) {
>> >> >> > > > >> > >> >  		bq->dev_rx = dev_rx;
>> >> >> > > > >> > >> > +		bq->xdp_prog = xdp_prog;    
>> >> >> > > > >> > >> bp->xdp_prog is assigned here and could be used later in bq_xmit_all().
>> >> >> > > > >> > >> How is bq->xdp_prog protected? Are they all under one rcu_read_lock()?
>> >> >> > > > >> > >> It is not very obvious after taking a quick look at xdp_do_flush[_map].
>> >> >> > > > >> > >> 
>> >> >> > > > >> > >> e.g. what if the devmap elem gets deleted.    
>> >> >> > > > >> > >
>> >> >> > > > >> > > Jesper knows better than me. From my veiw, based on the description of
>> >> >> > > > >> > > __dev_flush():
>> >> >> > > > >> > >
>> >> >> > > > >> > > On devmap tear down we ensure the flush list is empty before completing to
>> >> >> > > > >> > > ensure all flush operations have completed. When drivers update the bpf
>> >> >> > > > >> > > program they may need to ensure any flush ops are also complete.    
>> >> >> > > > >>
>> >> >> > > > >> AFAICT, the bq->xdp_prog is not from the dev. It is from a devmap's elem.
>> >> >> > 
>> >> >> > The bq->xdp_prog comes form the devmap "dev" element, and it is stored
>> >> >> > in temporarily in the "bq" structure that is only valid for this
>> >> >> > softirq NAPI-cycle.  I'm slightly worried that we copied this pointer
>> >> >> > the the xdp_prog here, more below (and Q for Paul).
>> >> >> > 
>> >> >> > > > >> > 
>> >> >> > > > >> > Yeah, drivers call xdp_do_flush() before exiting their NAPI poll loop,
>> >> >> > > > >> > which also runs under one big rcu_read_lock(). So the storage in the
>> >> >> > > > >> > bulk queue is quite temporary, it's just used for bulking to increase
>> >> >> > > > >> > performance :)    
>> >> >> > > > >>
>> >> >> > > > >> I am missing the one big rcu_read_lock() part.  For example, in i40e_txrx.c,
>> >> >> > > > >> i40e_run_xdp() has its own rcu_read_lock/unlock().  dst->xdp_prog used to run
>> >> >> > > > >> in i40e_run_xdp() and it is fine.
>> >> >> > > > >> 
>> >> >> > > > >> In this patch, dst->xdp_prog is run outside of i40e_run_xdp() where the
>> >> >> > > > >> rcu_read_unlock() has already done.  It is now run in xdp_do_flush_map().
>> >> >> > > > >> or I missed the big rcu_read_lock() in i40e_napi_poll()?
>> >> >> > > > >>
>> >> >> > > > >> I do see the big rcu_read_lock() in mlx5e_napi_poll().  
>> >> >> > > > >
>> >> >> > > > > I believed/assumed xdp_do_flush_map() was already protected under an
>> >> >> > > > > rcu_read_lock.  As the devmap and cpumap, which get called via
>> >> >> > > > > __dev_flush() and __cpu_map_flush(), have multiple RCU objects that we
>> >> >> > > > > are operating on.  
>> >> >> > >
>> >> >> > > What other rcu objects it is using during flush?
>> >> >> > 
>> >> >> > Look at code:
>> >> >> >  kernel/bpf/cpumap.c
>> >> >> >  kernel/bpf/devmap.c
>> >> >> > 
>> >> >> > The devmap is filled with RCU code and complicated take-down steps.  
>> >> >> > The devmap's elements are also RCU objects and the BPF xdp_prog is
>> >> >> > embedded in this object (struct bpf_dtab_netdev).  The call_rcu
>> >> >> > function is __dev_map_entry_free().
>> >> >> > 
>> >> >> > 
>> >> >> > > > > Perhaps it is a bug in i40e?  
>> >> >> > >
>> >> >> > > A quick look into ixgbe falls into the same bucket.
>> >> >> > > didn't look at other drivers though.
>> >> >> > 
>> >> >> > Intel driver are very much in copy-paste mode.
>> >> >> >  
>> >> >> > > > >
>> >> >> > > > > We are running in softirq in NAPI context, when xdp_do_flush_map() is
>> >> >> > > > > call, which I think means that this CPU will not go-through a RCU grace
>> >> >> > > > > period before we exit softirq, so in-practice it should be safe.  
>> >> >> > > > 
>> >> >> > > > Yup, this seems to be correct: rcu_softirq_qs() is only called between
>> >> >> > > > full invocations of the softirq handler, which for networking is
>> >> >> > > > net_rx_action(), and so translates into full NAPI poll cycles.  
>> >> >> > >
>> >> >> > > I don't know enough to comment on the rcu/softirq part, may be someone
>> >> >> > > can chime in.  There is also a recent napi_threaded_poll().
>> >> >> > 
>> >> >> > CC added Paul. (link to patch[1][2] for context)
>> >> >> Updated Paul's email address.
>> >> >> 
>> >> >> > 
>> >> >> > > If it is the case, then some of the existing rcu_read_lock() is unnecessary?
>> >> >> > 
>> >> >> > Well, in many cases, especially depending on how kernel is compiled,
>> >> >> > that is true.  But we want to keep these, as they also document the
>> >> >> > intend of the programmer.  And allow us to make the kernel even more
>> >> >> > preempt-able in the future.
>> >> >> > 
>> >> >> > > At least, it sounds incorrect to only make an exception here while keeping
>> >> >> > > other rcu_read_lock() as-is.
>> >> >> > 
>> >> >> > Let me be clear:  I think you have spotted a problem, and we need to
>> >> >> > add rcu_read_lock() at least around the invocation of
>> >> >> > bpf_prog_run_xdp() or before around if-statement that call
>> >> >> > dev_map_bpf_prog_run(). (Hangbin please do this in V8).
>> >> >> > 
>> >> >> > Thank you Martin for reviewing the code carefully enough to find this
>> >> >> > issue, that some drivers don't have a RCU-section around the full XDP
>> >> >> > code path in their NAPI-loop.
>> >> >> > 
>> >> >> > Question to Paul.  (I will attempt to describe in generic terms what
>> >> >> > happens, but ref real-function names).
>> >> >> > 
>> >> >> > We are running in softirq/NAPI context, the driver will call a
>> >> >> > bq_enqueue() function for every packet (if calling xdp_do_redirect) ,
>> >> >> > some driver wrap this with a rcu_read_lock/unlock() section (other have
>> >> >> > a large RCU-read section, that include the flush operation).
>> >> >> > 
>> >> >> > In the bq_enqueue() function we have a per_cpu_ptr (that store the
>> >> >> > xdp_frame packets) that will get flushed/send in the call
>> >> >> > xdp_do_flush() (that end-up calling bq_xmit_all()).  This flush will
>> >> >> > happen before we end our softirq/NAPI context.
>> >> >> > 
>> >> >> > The extension is that the per_cpu_ptr data structure (after this patch)
>> >> >> > store a pointer to an xdp_prog (which is a RCU object).  In the flush
>> >> >> > operation (which we will wrap with RCU-read section), we will use this
>> >> >> > xdp_prog pointer.   I can see that it is in-principle wrong to pass
>> >> >> > this-pointer between RCU-read sections, but I consider this safe as we
>> >> >> > are running under softirq/NAPI and the per_cpu_ptr is only valid in
>> >> >> > this short interval.
>> >> >> > 
>> >> >> > I claim a grace/quiescent RCU cannot happen between these two RCU-read
>> >> >> > sections, but I might be wrong? (especially in the future or for RT).
>> >> >
>> >> > If I am reading this correctly (ha!), a very high-level summary of the
>> >> > code in question is something like this:
>> >> >
>> >> > 	void foo(void)
>> >> > 	{
>> >> > 		local_bh_disable();
>> >> >
>> >> > 		rcu_read_lock();
>> >> > 		p = rcu_dereference(gp);
>> >> > 		do_something_with(p);
>> >> > 		rcu_read_unlock();
>> >> >
>> >> > 		do_something_else();
>> >> >
>> >> > 		rcu_read_lock();
>> >> > 		do_some_other_thing(p);
>> >> > 		rcu_read_unlock();
>> >> >
>> >> > 		local_bh_enable();
>> >> > 	}
>> >> >
>> >> > 	void bar(struct blat *new_gp)
>> >> > 	{
>> >> > 		struct blat *old_gp;
>> >> >
>> >> > 		spin_lock(my_lock);
>> >> > 		old_gp = rcu_dereference_protected(gp, lock_held(my_lock));
>> >> > 		rcu_assign_pointer(gp, new_gp);
>> >> > 		spin_unlock(my_lock);
>> >> > 		synchronize_rcu();
>> >> > 		kfree(old_gp);
>> >> > 	}
>> >> 
>> >> Yeah, something like that (the object is freed using call_rcu() - but I
>> >> think that's equivalent, right?). And the question is whether we need to
>> >> extend foo() so that is has one big rcu_read_lock() that covers the
>> >> whole lifetime of p.
>> >
>> > Yes, use of call_rcu() is an asynchronous version of synchronize_rcu().
>> > In fact, synchronize_rcu() is implemented in terms of call_rcu().  ;-)
>> 
>> Right, gotcha!
>> 
>> >> > I need to check up on -rt.
>> >> >
>> >> > But first... In recent mainline kernels, the local_bh_disable() region
>> >> > will look like one big RCU read-side critical section.  But don't try
>> >> > this prior to v4.20!!!  In v4.19 and earlier, you would need to use
>> >> > both synchronize_rcu() and synchronize_rcu_bh() to make this work, or,
>> >> > for less latency, synchronize_rcu_mult(call_rcu, call_rcu_bh).
>> >> 
>> >> OK. Variants of this code has been around since before then, but I
>> >> honestly have no idea what it looked like back then exactly...
>> >
>> > I know that feeling...
>> >
>> >> > Except that in that case, why not just drop the inner rcu_read_unlock()
>> >> > and rcu_read_lock() pair?  Awkward function boundaries or some such?
>> >> 
>> >> Well if we can just treat such a local_bh_disable()/enable() pair as the
>> >> equivalent of rcu_read_lock()/unlock() then I suppose we could just get
>> >> rid of the inner ones. What about tools like lockdep; do they understand
>> >> this, or are we likely to get complaints if we remove it?
>> >
>> > If you just got rid of the first rcu_read_unlock() and the second
>> > rcu_read_lock() in the code above, lockdep will understand.
>> 
>> Right, but doing so entails going through all the drivers, which is what
>> we're trying to avoid :)
>
> I was afraid of that...  ;-)
>
>> > However, if you instead get rid of -all- of the rcu_read_lock() and
>> > rcu_read_unlock() invocations in the code above, you would need to let
>> > lockdep know by adding rcu_read_lock_bh_held().  So instead of this:
>> >
>> > 	p = rcu_dereference(gp);
>> >
>> > You would do this:
>> >
>> > 	p = rcu_dereference_check(gp, rcu_read_lock_bh_held());
>> >
>> > This would be needed for mainline, regardless of -rt.
>> 
>> OK. And as far as I can tell this is harmless for code paths that call
>> the same function but from a regular rcu_read_lock()-protected section
>> instead from a bh-disabled section, right?
>
> That is correct.  That rcu_dereference_check() invocation will make
> lockdep be OK with rcu_read_lock() or with softirq being disabled.
> Or both, for that matter.

OK, great, thank you for confirming my understanding!

>> What happens, BTW, if we *don't* get rid of all the existing
>> rcu_read_lock() sections? Going back to your foo() example above, what
>> we're discussing is whether to add that second rcu_read_lock() around
>> do_some_other_thing(p). I.e., the first one around the rcu_dereference()
>> is already there (in the particular driver we're discussing), and the
>> local_bh_disable/enable() pair is already there. AFAICT from our
>> discussion, there really is not much point in adding that second
>> rcu_read_lock/unlock(), is there?
>
> From an algorithmic point of view, the second rcu_read_lock()
> and rcu_read_unlock() are redundant.  Of course, there are also
> software-engineering considerations, including copy-pasta issues.
>
>> And because that first rcu_read_lock() around the rcu_dereference() is
>> already there, lockdep is not likely to complain either, so we're
>> basically fine? Except that the code is somewhat confusing as-is, of
>> course; i.e., we should probably fix it but it's not terribly urgent. Or?
>
> I am concerned about copy-pasta-induced bugs.  Someone looks just at
> the code, fails to note the fact that softirq is disabled throughout,
> and decides that leaking a pointer from one RCU read-side critical
> section to a later one is just fine.  :-/

Yup, totally agreed that we need to fix this for the sake of the humans
reading the code; just wanted to make sure my understanding was correct
that we don't strictly need to do anything as far as the machines
executing it are concerned :)

>> Hmm, looking at it now, it seems not all the lookup code is actually
>> doing rcu_dereference() at all, but rather just a plain READ_ONCE() with
>> a comment above it saying that RCU ensures objects won't disappear[0];
>> so I suppose we're at least safe from lockdep in that sense :P - but we
>> should definitely clean this up.
>> 
>> [0] Exhibit A: https://elixir.bootlin.com/linux/latest/source/kernel/bpf/devmap.c#L391
>
> That use of READ_ONCE() will definitely avoid lockdep complaints,
> including those complaints that point out bugs.  It also might get you
> sparse complaints if the RCU-protected pointer is marked with __rcu.

It's not; it's the netdev_map member of this struct:

struct bpf_dtab {
	struct bpf_map map;
	struct bpf_dtab_netdev **netdev_map; /* DEVMAP type only */
	struct list_head list;

	/* these are only used for DEVMAP_HASH type maps */
	struct hlist_head *dev_index_head;
	spinlock_t index_lock;
	unsigned int items;
	u32 n_buckets;
};

Will adding __rcu to such a dynamic array member do the right thing when
paired with rcu_dereference() on array members (i.e., in place of the
READ_ONCE in the code linked above)?

Also, while you're being so nice about confirming my understanding of
things: I always understood the point of rcu_dereference() (and __rcu on
struct members) to be annotations that document the lifetime
expectations of the object being pointed to, rather than a functional
change vs READ_ONCE()? Documentation that the static checkers can turn
into warnings, of course, but totally transparent in terms of the
generated code. Right? :)

>> >> > Especially given that if this works on -rt, it is probably because
>> >> > their variant of do_softirq() holds rcu_read_lock() across each
>> >> > softirq handler invocation. They do something similar for rwlocks.
>> >> 
>> >> Right. Guess we'll wait for your confirmation of that, then. Thanks! :)
>> >
>> > Looking at v5.11.4-rt11...
>> >
>> > And __local_bh_disable_ip() has added the required rcu_read_lock(),
>> > so dropping all the rcu_read_lock() and rcu_read_unlock() calls would
>> > do the right thing in -rt.  And lockdep would understand without the
>> > rcu_read_lock_bh_held(), but that is still required for mainline.
>> 
>> Great, thanks for checking!
>> 
>> So this brings to mind another question: Are there any performance
>> implications to nesting rcu_read_locks() inside each other? One
>> thing that would be fairly easy to do (in terms of how much code we have
>> to touch) is to just add a top-level rcu_read_lock() around the
>> napi_poll() call in the core dev code, thus making -rt and mainline
>> equivalent in that respect. Also, this would make it obvious that all
>> the RCU usage inside of NAPI is safe, without having to know about
>> bh_disable() and all that. But we obviously don't want to do that if it
>> is going to slow things down; WDYT?
>
> Both rcu_read_lock() and rcu_read_unlock() are quite lightweight (zero for
> CONFIG_PREEMPT=n and about two nanoseconds per pair for CONFIG_PREEMPT=y
> on 2GHz x86) and can be nested quite deeply.  So that approach should
> be fine from that viewpoint.

OK, that may be fine, then. Guess I'll try it and benchmark (and compare
with the rcu_dereference_check() approach).

> However, remaining in a single RCU read-side critical section forever
> will eventually OOM the system, so the code should periodically exit
> its top-level RCU read-side critical section, say, every few tens of
> milliseconds.

Yup, NAPI already does this (there's a poll budget), so that should be
fine.

-Toke

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