Breno Leitao <lei...@debian.org> writes:
> Hello Michael,
>
> On Mon, Aug 14, 2017 at 11:00:07PM +1000, Michael Ellerman wrote:
>> Breno Leitao <lei...@debian.org> writes:
>> > @@ -2231,6 +2232,19 @@ static void xmon_rawdump (unsigned long adrs, long
>> > ndump)
>> > printf("\n");
>> > }
>> >
>> > +static void dump_tracing(void)
>> > +{
>> > + int c;
>> > +
>> > + c = inchar();
>> > + if (c == 'c')
>> > + ftrace_dump(DUMP_ORIG);
>> > + else
>> > + ftrace_dump(DUMP_ALL);
>> > +
>> > + tracing_on();
>> > +}
>>
>> Thinking about this some more, two things that would make this *really*
>> useful.
>>
>> Firstly, it would be great if we could dump the buffer for *another*
>> CPU.
>
> Well, you can do it with this new 'dtc' option on xmon. You just need to
> change to that CPU prior to call 'dtc'.
But that's the problem. If the other CPU is stuck then you can't change
to it in xmon.
If it *isn't* stuck, then you can just change to it and backtrace like
normal.
eg, if you use the HARDLOCKUP lkdtm test:
# cd /sys/kernel/debug/provoke-crash
# echo HARDLOCKUP > DIRECT
sysrq: SysRq : Entering xmon
cpu 0x1: Vector: 501 (Hardware Interrupt) at [c0000000fe9af9a0]
pc: c0000000000f15d0: plpar_hcall_norets+0x1c/0x28
lr: c000000000daaf04: check_and_cede_processor+0x34/0x50
sp: c0000000fe9afc20
msr: 8000000000009033
current = 0xc0000000fe95a200
paca = 0xc00000000fd40580 softe: 0 irq_happened: 0x09
pid = 0, comm = swapper/1
Linux version 4.13.0-rc2-gcc-6.3.1-00101-ged49f7fd6438
(mich...@ka3.ozlabs.ibm.com) (gcc version 6.3.1 20170214 (Custom
e9096cb27f4bd642)) #455 SMP Mon Aug 14 22:19:37 AEST 2017
enter ? for help
[link register ] c000000000daaf04 check_and_cede_processor+0x34/0x50
[c0000000fe9afc20] c000000000daaef0 check_and_cede_processor+0x20/0x50
(unreliable)
[c0000000fe9afc80] c000000000daaf94 shared_cede_loop+0x74/0x2b0
[c0000000fe9afcd0] c000000000da6fa0 cpuidle_enter_state+0xe0/0x6b0
[c0000000fe9afd50] c0000000001ff17c call_cpuidle+0x7c/0x110
[c0000000fe9afd90] c0000000001ff800 do_idle+0x350/0x460
[c0000000fe9afe20] c0000000001ffbe8 cpu_startup_entry+0x38/0x40
[c0000000fe9afe50] c000000000060108 start_secondary+0x528/0xb90
[c0000000fe9aff90] c00000000000af6c start_secondary_prolog+0x10/0x14
1:mon> c
cpus stopped: 0x0-0x5 0x7-0xf
1:mon>
1:mon> c 6
cpu 0x6 isn't in xmon
Notice that CPU 6 hasn't called in, so we can't switch to it, and we
have no (easy) way of working out where it is.
If we could dump the ftrace buffer for CPU 6 from another CPU, we'd be
able to see eg:
6) | SyS_write() {
6) | __fdget_pos() {
6) 0.088 us | __fget_light();
6) 1.168 us | }
6) | vfs_write() {
6) 0.092 us | rw_verify_area();
6) 0.062 us | __sb_start_write();
6) | __vfs_write() {
...
6) | lkdtm_do_action() {
6) | lkdtm_HARDLOCKUP() {
---------------------------------
Which would be helpful.
cheers
