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: Fri, 16 Apr 2021 12:03:41 +0200

Martin KaFai Lau <[email-masked]> writes:

> 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.
>> >> 
>> >> > 
>> >> > 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?

The bq_enqueue() function in cpumap.c puts the 'bq' pointer onto the
flush_list, and 'bq' lives inside struct bpf_cpu_map_entry, so that's a
reference to the map entry as well.

The devmap function used to work the same way, until we changed it in
75ccae62cb8d ("xdp: Move devmap bulk queue into struct net_device").

>> > Perhaps it is a bug in i40e?
> A quick look into ixgbe falls into the same bucket.
> didn't look at other drivers though.
>
>> >
>> > 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().
>
> If it is the case, then some of the existing rcu_read_lock() is unnecessary?
> At least, it sounds incorrect to only make an exception here while keeping
> other rcu_read_lock() as-is.

I'd tend to agree that the correct thing to do is to fix any affected
drivers so there's a wide rcu_read_lock() around the full xdp+flush. If
nothing else, this serves as an annotation for the expected lifetime of
the objects involved.

However, given that this is not a new issue, I don't think it should be
holding up this patch series... We can start a new conversation on what
the right way to fix this is - and maybe bring in Paul for advice on the
RCU side? WDYT?

-Toke