Subject: Re: [PATCHv7 bpf-next 1/4] bpf: run devmap xdp_prog on flush instead of bulk enqueue

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Sat, 17 Apr 2021 14:27:19 +0200

"Paul E. McKenney" <[email-masked]> 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 <[email-masked]> wrote:
>> > 
>> > > On Thu, Apr 15, 2021 at 10:29:40PM +0200, Toke Høiland-Jørgensen wrote:
>> > > > Jesper Dangaard Brouer <[email-masked]> writes:
>> > > >   
>> > > > > On Thu, 15 Apr 2021 10:35:51 -0700
>> > > > > Martin KaFai Lau <[email-masked]> wrote:
>> > > > >  
>> > > > >> On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke Høiland-Jørgensen wrote:  
>> > > > >> > Hangbin Liu <[email-masked]> 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.

> 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...

> 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?

> 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! :)

-Toke