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SubjectRe: [PATCH v3] tracing: Ignore mmiotrace from kernel commandline
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On 09/11/2017 07:23 PM, Steven Rostedt wrote:
> On Mon, 11 Sep 2017 14:31:34 +0800
> "Ziqian SUN (Zamir)" <zsun@redhat.com> wrote:
>
>> On 09/11/2017 02:26 PM, Ziqian SUN (Zamir) wrote:
>>> From: "Ziqian SUN (Zamir)" <zsun@redhat.com>
>>>
>>> The mmiotrace tracer cannot be enabled with ftrace=mmiotrace in kernel
>>> commandline. With this patch, noboot is added to the tracer struct,
>>> and when system boot with a tracer that has noboot=true, it will print
>>> out a warning message and continue booting.
>>>
>>> Signed-off-by: Ziqian SUN (Zamir) <zsun@redhat.com>
>>> --
>>> v1 -> v2 : remove unessential comment
>>> v2 -> v3 : Use tracer struct instead of a separate list to store noboot
>>> ---
>>> kernel/trace/trace.c | 7 +++++++
>>> kernel/trace/trace.h | 2 ++
>>> kernel/trace/trace_mmiotrace.c | 1 +
>>> 3 files changed, 10 insertions(+)
>>>
>>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>>> index 5360b7a..48c474f 100644
>>> --- a/kernel/trace/trace.c
>>> +++ b/kernel/trace/trace.c
>>> @@ -5358,6 +5358,13 @@ static int tracing_set_tracer(struct trace_array *tr, const char *buf)
>>> if (t == tr->current_trace)
>>> goto out;
>>>
>>> + /* Some tracers won't work on kernel command line */
>>> + if (system_state < SYSTEM_RUNNING && t->noboot) {
>>> + pr_warn("Tracer '%s' is not allowed on command line, ignored\n",
>>> + t->name);
>> I feel the core trace printed by WARN is not so meaningful, so I use
>> pr_warn instead.
>
> I'm fine with pr_warn, but I'm curious to what you mean by "not so
> meaningful"? A WARN() will cause a dump stack, which usually shows up
> as a bug in systems and more likely to be seen. But if someone is
> adding this to the kernel command line and it's not working, they
> should be looking for the tracer name within the dmesg anyway.
>

Following is the log I tried with WARN before. With WARN we have both
the warning message, and the stack trace. The stack trace goes into
stacing_set_tracer, but not showing mmiotrace. I mean, this is useful
for debugging, but maybe confusing for users who is willing to report
bugs but not a kernel geek. With pr_warn we already indicate mmiotrace
is ignored, and is neat.

Just my cents.

[ 7.134149] Starting tracer 'mmiotrace'
[ 7.134151] Tracer 'mmiotrace' is not allowed on command line
[ 7.134162] ------------[ cut here ]------------
[ 7.134171] WARNING: CPU: 6 PID: 1 at kernel/trace/trace.c:5354
tracing_set_tracer+0x168/0x1b0
[ 7.134171] Modules linked in:
[ 7.134176] CPU: 6 PID: 1 Comm: swapper/0 Not tainted
4.13.0-1.mmiotracev3.fc26.x86_64 #1
[ 7.134178] Hardware name: HP ProLiant DL380p Gen8, BIOS P70 09/18/2013
[ 7.134179] task: ffff94b7310d8000 task.stack: ffffaadc83144000
[ 7.134183] RIP: 0010:tracing_set_tracer+0x168/0x1b0
[ 7.134185] RSP: 0000:ffffaadc83147da0 EFLAGS: 00010286
[ 7.134187] RAX: 0000000000000031 RBX: ffffffffad04e340 RCX:
0000000000000000
[ 7.134189] RDX: 0000000000000000 RSI: 0000000000000002 RDI:
0000000000000246
[ 7.134190] RBP: ffffaadc83147dc8 R08: 0000000000000031 R09:
ffffffffad314f00
[ 7.134191] R10: ffffaadc83147d30 R11: 0000000000000000 R12:
0000000000000000
[ 7.134193] R13: ffffffffacc935f3 R14: ffffffffacc935f3 R15:
ffffffffacef46c0
[ 7.134196] FS: 0000000000000000(0000) GS:ffff94b732e00000(0000)
knlGS:0000000000000000
[ 7.134197] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 7.134199] CR2: 0000000000000000 CR3: 00000005f5e09000 CR4:
00000000000406e0
[ 7.134201] Call Trace:
[ 7.134210] ? set_debug_rodata+0x17/0x17
[ 7.134213] register_tracer+0x1a1/0x1c2
[ 7.134216] ? stack_trace_init+0xb5/0xb5
[ 7.134218] init_mmio_trace+0x10/0x12
[ 7.134222] do_one_initcall+0x50/0x190
[ 7.134225] kernel_init_freeable+0x1a8/0x245
[ 7.134231] ? rest_init+0xc0/0xc0
[ 7.134234] kernel_init+0xe/0x101
[ 7.134238] ret_from_fork+0x25/0x30
[ 7.134240] Code: ff e8 6d d0 ff ff 85 c0 41 89 c4 79 8e eb b7 80 bb
9b 00 00 00 00 0f 84 26 ff ff ff 4c 89 ee 48 c7 c7 58 1e c9 ac e8 99 a0
f8 ff <0f> ff eb 97 48 8b 35 9d 81 d7 00 ba ff ff ff ff 4c 89 ff e8 10
[ 7.134279] ---[ end trace 978ff599b6e516bc ]---

> Also, I'm currently at OSS in LA and hopefully I don't lose this patch.
> If you don't see anything by next Monday from me, feel free to send me a
> reminder ping.
>
> -- Steve
>

--
Ziqian SUN (Zamir)
9F Raycom office (NAY)
Red Hat Software (Beijing) Co.,Ltd
IRC: zsun (internal and freenode)
Tel: +86 10 65627458
GPG : 1D86 6D4A 49CE 4BBD 72CF FCF5 D856 6E11 F2A0 525E

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