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

[Martin KaFai Lau <kafai@fb.com>]

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@fb.com> wrote:
> 
> > On Thu, Apr 15, 2021 at 10:29:40PM +0200, Toke Høiland-Jørgensen wrote:
> > > Jesper Dangaard Brouer <brouer@redhat.com> writes:
> > >   
> > > > On Thu, 15 Apr 2021 10:35:51 -0700
> > > > Martin KaFai Lau <kafai@fb.com> wrote:
> > > >  
> > > >> On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke Høiland-Jørgensen wrote:  
> > > >> > Hangbin Liu <liuhangbin@gmail.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).
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer 
> 
> [1] https://lore.kernel.org/netdev/20210414122610.4037085-2-liuhangbin@gmail.com/
> [2] https://patchwork.kernel.org/project/netdevbpf/patch/20210414122610.4037085-2-liuhangbin@gmail.com/
>