linux-kernel - Re: [PATCH bpf v2] bpf: preallocate a perf_sample

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Date:   Mon, 3 Jun 2019 22:58:20 +0000
From:   Matt Mullins <mmullins@...com>
To:     "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Andrew Hall <hall@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf v2] bpf: preallocate a perf_sample_data per event fd

On Mon, 2019-06-03 at 15:22 +0200, Daniel Borkmann wrote:
> On 06/03/2019 03:08 PM, Daniel Borkmann wrote:
> > On 06/01/2019 12:37 AM, Matt Mullins wrote:
> > > It is possible that a BPF program can be called while another BPF
> > > program is executing bpf_perf_event_output.  This has been observed with
> > > I/O completion occurring as a result of an interrupt:
> > > 
> > > 	bpf_prog_247fd1341cddaea4_trace_req_end+0x8d7/0x1000
> > > 	? trace_call_bpf+0x82/0x100
> > > 	? sch_direct_xmit+0xe2/0x230
> > > 	? blk_mq_end_request+0x1/0x100
> > > 	? blk_mq_end_request+0x5/0x100
> > > 	? kprobe_perf_func+0x19b/0x240
> > > 	? __qdisc_run+0x86/0x520
> > > 	? blk_mq_end_request+0x1/0x100
> > > 	? blk_mq_end_request+0x5/0x100
> > > 	? kprobe_ftrace_handler+0x90/0xf0
> > > 	? ftrace_ops_assist_func+0x6e/0xe0
> > > 	? ip6_input_finish+0xbf/0x460
> > > 	? 0xffffffffa01e80bf
> > > 	? nbd_dbg_flags_show+0xc0/0xc0 [nbd]
> > > 	? blkdev_issue_zeroout+0x200/0x200
> > > 	? blk_mq_end_request+0x1/0x100
> > > 	? blk_mq_end_request+0x5/0x100
> > > 	? flush_smp_call_function_queue+0x6c/0xe0
> > > 	? smp_call_function_single_interrupt+0x32/0xc0
> > > 	? call_function_single_interrupt+0xf/0x20
> > > 	? call_function_single_interrupt+0xa/0x20
> > > 	? swiotlb_map_page+0x140/0x140
> > > 	? refcount_sub_and_test+0x1a/0x50
> > > 	? tcp_wfree+0x20/0xf0
> > > 	? skb_release_head_state+0x62/0xc0
> > > 	? skb_release_all+0xe/0x30
> > > 	? napi_consume_skb+0xb5/0x100
> > > 	? mlx5e_poll_tx_cq+0x1df/0x4e0
> > > 	? mlx5e_poll_tx_cq+0x38c/0x4e0
> > > 	? mlx5e_napi_poll+0x58/0xc30
> > > 	? mlx5e_napi_poll+0x232/0xc30
> > > 	? net_rx_action+0x128/0x340
> > > 	? __do_softirq+0xd4/0x2ad
> > > 	? irq_exit+0xa5/0xb0
> > > 	? do_IRQ+0x7d/0xc0
> > > 	? common_interrupt+0xf/0xf
> > > 	</IRQ>
> > > 	? __rb_free_aux+0xf0/0xf0
> > > 	? perf_output_sample+0x28/0x7b0
> > > 	? perf_prepare_sample+0x54/0x4a0
> > > 	? perf_event_output+0x43/0x60
> > > 	? bpf_perf_event_output_raw_tp+0x15f/0x180
> > > 	? blk_mq_start_request+0x1/0x120
> > > 	? bpf_prog_411a64a706fc6044_should_trace+0xad4/0x1000
> > > 	? bpf_trace_run3+0x2c/0x80
> > > 	? nbd_send_cmd+0x4c2/0x690 [nbd]
> > > 
> > > This also cannot be alleviated by further splitting the per-cpu
> > > perf_sample_data structs (as in commit 283ca526a9bd ("bpf: fix
> > > corruption on concurrent perf_event_output calls")), as a raw_tp could
> > > be attached to the block:block_rq_complete tracepoint and execute during
> > > another raw_tp.  Instead, keep a pre-allocated perf_sample_data
> > > structure per perf_event_array element and fail a bpf_perf_event_output
> > > if that element is concurrently being used.
> > > 
> > > Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
> > > Signed-off-by: Matt Mullins <mmullins@...com>
> > 
> > You do not elaborate why is this needed for all the networking programs that
> > use this functionality. The bpf_misc_sd should therefore be kept as-is. There
> > cannot be nested occurrences there (xdp, tc ingress/egress). Please explain why
> > non-tracing should be affected here...

If these are invariably non-nested, I can easily keep bpf_misc_sd when
I resubmit.  There was no technical reason other than keeping the two
codepaths as similar as possible.

What resource gives you worry about doing this for the networking
codepath?

> Aside from that it's also really bad to miss events like this as exporting
> through rb is critical. Why can't you have a per-CPU counter that selects a
> sample data context based on nesting level in tracing? (I don't see a discussion
> of this in your commit message.)

This change would only drop messages if the same perf_event is
attempted to be used recursively (i.e. the same CPU on the same
PERF_EVENT_ARRAY map, as I haven't observed anything use index !=
BPF_F_CURRENT_CPU in testing).

I'll try to accomplish the same with a percpu nesting level and
allocating 2 or 3 perf_sample_data per cpu.  I think that'll solve the
same problem -- a local patch keeping track of the nesting level is how
I got the above stack trace, too.