netdev - Re: xdpsock poll with 5.2.21rt kernel

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Message-ID: <CAD56B7d3tyWfweZYogywebTcTvZQqK433e5w0GeahHJRzS2cDg@mail.gmail.com>
Date:   Mon, 2 Dec 2019 12:11:08 -0500
From:   Paul Thomas <pthomas8589@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, xdp-newbies@...r.kernel.org,
        bpf@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: xdpsock poll with 5.2.21rt kernel

> >
> > Well, it does complain (report below), but I'm not sure it's related.
> > The other thing I tried was the AF_XDP example here:
> > https://github.com/xdp-project/xdp-tutorial/tree/master/advanced03-AF_XDP
> >
> > With this example poll() always seems to block correctly, so I think
> > maybe there is something wrong with the xdpsock_user.c example or how
> > I'm using it.
> >
> > [  259.591480] BUG: assuming atomic context at net/core/ptp_classifier.c:106
> > [  259.591488] in_atomic(): 0, irqs_disabled(): 0, pid: 953, name: irq/22-eth%d
> > [  259.591494] CPU: 0 PID: 953 Comm: irq/22-eth%d Tainted: G        WC
> >        5.
> >
> >                         2.21-rt13-00016-g93898e751d0e #90
> > [  259.591499] Hardware name: Enclustra XU5 SOM (DT)
> > [  259.591501] Call trace:
> > [  259.591503] dump_backtrace (/arch/arm64/kernel/traps.c:94)
> > [  259.591514] show_stack (/arch/arm64/kernel/traps.c:151)
> > [  259.591520] dump_stack (/lib/dump_stack.c:115)
> > [  259.591526] __cant_sleep (/kernel/sched/core.c:6386)
> > [  259.591531] ptp_classify_raw (/./include/linux/compiler.h:194
>
> Is this the only splat? Nothing more? I would expect something at boot
> time, too.
I should have expanded more. This seems to happen every second
starting at boot in ptp_classifier.c regardless of if I'm doing
anything with BPF.

>
> So this part expects disabled preemption. Other invocations disable
> preemption. The whole BPF part is currently not working on -RT.
OK, so I should expect more issues as we play with AF_XDP? An
application based on the other example [1] is at least running.
Preempt-rt + AF_XDP seems like an awesome combination, so I hopefully
any BPF issues can be resolved.

thanks,
Paul

[1] https://github.com/xdp-project/xdp-tutorial/tree/master/advanced03-AF_XDP