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Thu, 15 Apr 2021 18:21:50 +0000 (UTC) Received: from carbon (unknown [10.36.110.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6DE4C5C3FD; Thu, 15 Apr 2021 18:21:33 +0000 (UTC) Date: Thu, 15 Apr 2021 20:21:32 +0200 From: Jesper Dangaard Brouer To: Martin KaFai Lau Cc: Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?= , Hangbin Liu , , , Jiri Benc , Eelco Chaudron , , Daniel Borkmann , Lorenzo Bianconi , David Ahern , Andrii Nakryiko , Alexei Starovoitov , John Fastabend , Maciej Fijalkowski , =?UTF-8?B?QmrDtnJuIFQ=?= =?UTF-8?B?w7ZwZWw=?= , brouer@redhat.com Subject: Re: [PATCHv7 bpf-next 1/4] bpf: run devmap xdp_prog on flush instead of bulk enqueue Message-ID: <20210415202132.7b5e8d0d@carbon> In-Reply-To: <20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com> References: <20210414122610.4037085-1-liuhangbin@gmail.com> <20210414122610.4037085-2-liuhangbin@gmail.com> <20210415001711.dpbt2lej75ry6v7a@kafai-mbp.dhcp.thefacebook.com> <20210415023746.GR2900@Leo-laptop-t470s> <87o8efkilw.fsf@toke.dk> <20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, 15 Apr 2021 10:35:51 -0700 Martin KaFai Lau wrote: > On Thu, Apr 15, 2021 at 11:22:19AM +0200, Toke H=C3=B8iland-J=C3=B8rgense= n wrote: > > Hangbin Liu writes: > > =20 > > > On Wed, Apr 14, 2021 at 05:17:11PM -0700, Martin KaFai Lau wrote: =20 > > >> > static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags) > > >> > { > > >> > struct net_device *dev =3D bq->dev; > > >> > - int sent =3D 0, err =3D 0; > > >> > + int sent =3D 0, drops =3D 0, err =3D 0; > > >> > + unsigned int cnt =3D bq->count; > > >> > + int to_send =3D cnt; > > >> > int i; > > >> > =20 > > >> > - if (unlikely(!bq->count)) > > >> > + if (unlikely(!cnt)) > > >> > return; > > >> > =20 > > >> > - for (i =3D 0; i < bq->count; i++) { > > >> > + for (i =3D 0; i < cnt; i++) { > > >> > struct xdp_frame *xdpf =3D bq->q[i]; > > >> > =20 > > >> > prefetch(xdpf); > > >> > } > > >> > =20 > > >> > - sent =3D dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, fl= ags); > > >> > + if (bq->xdp_prog) { =20 > > >> bq->xdp_prog is used here > > >> =20 > > >> > + to_send =3D dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev); > > >> > + if (!to_send) > > >> > + goto out; > > >> > + > > >> > + drops =3D cnt - to_send; > > >> > + } > > >> > + =20 > > >>=20 > > >> [ ... ] > > >> =20 > > >> > 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 =3D this_cpu_ptr(&dev_flush_list); > > >> > struct xdp_dev_bulk_queue *bq =3D this_cpu_ptr(dev->xdp_bulkq); > > >> > @@ -412,18 +466,22 @@ static void bq_enqueue(struct net_device *de= v, 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 =3D dev_rx; > > >> > + bq->xdp_prog =3D xdp_prog; =20 > > >> 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[_ma= p]. > > >>=20 > > >> e.g. what if the devmap elem gets deleted. =20 > > > > > > 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 completi= ng to > > > ensure all flush operations have completed. When drivers update the b= pf > > > program they may need to ensure any flush ops are also complete. =20 > > AFAICT, the bq->xdp_prog is not from the dev. It is from a devmap's elem. >=20 > >=20 > > 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 :) =20 > > 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. >=20 > 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. Perhaps it is a bug in i40e? 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. But to be correct I do think we need a rcu_read_lock() around this call. --=20 Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer