Messages in this thread | | | From | Doug Anderson <> | Date | Wed, 25 Sep 2019 13:03:02 -0700 | Subject | Re: [PATCH v2] kdb: Fix stack crawling on 'running' CPUs that aren't the master |
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Hi,
On Fri, Aug 30, 2019 at 7:52 AM Daniel Thompson <daniel.thompson@linaro.org> wrote: > > On Wed, Jul 31, 2019 at 11:37:32AM -0700, Douglas Anderson wrote: > > In kdb when you do 'btc' (back trace on CPU) it doesn't necessarily > > give you the right info. Specifically on many architectures > > (including arm64, where I tested) you can't dump the stack of a > > "running" process that isn't the process running on the current CPU. > > This can be seen by this: > > > > echo SOFTLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT > > # wait 2 seconds > > <sysrq>g > > > > Here's what I see now on rk3399-gru-kevin. I see the stack crawl for > > the CPU that handled the sysrq but everything else just shows me stuck > > in __switch_to() which is bogus: > > > > ====== > > > > [0]kdb> btc > > btc: cpu status: Currently on cpu 0 > > Available cpus: 0, 1-3(I), 4, 5(I) > > Stack traceback for pid 0 > > 0xffffff801101a9c0 0 0 1 0 R 0xffffff801101b3b0 *swapper/0 > > Call trace: > > dump_backtrace+0x0/0x138 > > ... > > kgdb_compiled_brk_fn+0x34/0x44 > > ... > > sysrq_handle_dbg+0x34/0x5c > > Stack traceback for pid 0 > > 0xffffffc0f175a040 0 0 1 1 I 0xffffffc0f175aa30 swapper/1 > > Call trace: > > __switch_to+0x1e4/0x240 > > 0xffffffc0f65616c0 > > Stack traceback for pid 0 > > 0xffffffc0f175d040 0 0 1 2 I 0xffffffc0f175da30 swapper/2 > > Call trace: > > __switch_to+0x1e4/0x240 > > 0xffffffc0f65806c0 > > Stack traceback for pid 0 > > 0xffffffc0f175b040 0 0 1 3 I 0xffffffc0f175ba30 swapper/3 > > Call trace: > > __switch_to+0x1e4/0x240 > > 0xffffffc0f659f6c0 > > Stack traceback for pid 1474 > > 0xffffffc0dde8b040 1474 727 1 4 R 0xffffffc0dde8ba30 bash > > Call trace: > > __switch_to+0x1e4/0x240 > > __schedule+0x464/0x618 > > 0xffffffc0dde8b040 > > Stack traceback for pid 0 > > 0xffffffc0f17b0040 0 0 1 5 I 0xffffffc0f17b0a30 swapper/5 > > Call trace: > > __switch_to+0x1e4/0x240 > > 0xffffffc0f65dd6c0 > > > > === > > > > The problem is that 'btc' eventually boils down to > > show_stack(task_struct, NULL); > > > > ...and show_stack() doesn't work for "running" CPUs because their > > registers haven't been stashed. > > > > On x86 things might work better (I haven't tested) because kdb has a > > special case for x86 in kdb_show_stack() where it passes the stack > > pointer to show_stack(). This wouldn't work on arm64 where the stack > > crawling function seems needs the "fp" and "pc", not the "sp" which is > > presumably why arm64's show_stack() function totally ignores the "sp" > > parameter. > > > > NOTE: we _can_ get a good stack dump for all the cpus if we manually > > switch each one to the kdb master and do a back trace. AKA: > > cpu 4 > > bt > > ...will give the expected trace. That's because now arm64's > > dump_backtrace will now see that "tsk == current" and go through a > > different path. > > > > In this patch I fix the problems by catching a request to stack crawl > > a task that's running on a CPU and then I ask that CPU to do the stack > > crawl. > > > > NOTE: this will (presumably) change what stack crawls are printed for > > x86 machines. Now kdb functions will show up in the stack crawl. > > Presumably this is OK but if it's not we can go back and add a special > > case for x86 again. > > > > Signed-off-by: Douglas Anderson <dianders@chromium.org> > > I think this approach can be made work but there are problems as things > exist today, see below. > > > > --- > > > > Changes in v2: > > - Totally new approach; now arch agnostic. > > > > kernel/debug/debug_core.c | 5 +++++ > > kernel/debug/debug_core.h | 1 + > > kernel/debug/kdb/kdb_bt.c | 44 ++++++++++++++++++++++++++++++--------- > > 3 files changed, 40 insertions(+), 10 deletions(-) > > > > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c > > index 5cc608de6883..a89c72714fe6 100644 > > --- a/kernel/debug/debug_core.c > > +++ b/kernel/debug/debug_core.c > > @@ -92,6 +92,8 @@ static int kgdb_use_con; > > bool dbg_is_early = true; > > /* Next cpu to become the master debug core */ > > int dbg_switch_cpu; > > +/* cpu number of slave we request a stack crawl of */ > > +int dbg_slave_dumpstack_cpu = -1; > > > > /* Use kdb or gdbserver mode */ > > int dbg_kdb_mode = 1; > > @@ -580,6 +582,9 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, > > atomic_xchg(&kgdb_active, cpu); > > break; > > } > > + } else if (dbg_slave_dumpstack_cpu == cpu) { > > Couldn't this be encoded in the exception state?
Yup, but it requires exporting the exception state from debug_core.c (or exporting a function that sets this). Ah, but below you said you wanted the whole stack crawling on a CPU moved to debug_core.c, so done.
> > + dump_stack(); > > + dbg_slave_dumpstack_cpu = -1; > > > } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) { > > if (!raw_spin_is_locked(&dbg_slave_lock)) > > goto return_normal; > > diff --git a/kernel/debug/debug_core.h b/kernel/debug/debug_core.h > > index b4a7c326d546..dca74d5caef2 100644 > > --- a/kernel/debug/debug_core.h > > +++ b/kernel/debug/debug_core.h > > @@ -62,6 +62,7 @@ extern int dbg_io_get_char(void); > > /* Switch from one cpu to another */ > > #define DBG_SWITCH_CPU_EVENT -123456 > > extern int dbg_switch_cpu; > > +extern int dbg_slave_dumpstack_cpu; > > > > /* gdbstub interface functions */ > > extern int gdb_serial_stub(struct kgdb_state *ks); > > diff --git a/kernel/debug/kdb/kdb_bt.c b/kernel/debug/kdb/kdb_bt.c > > index 7e2379aa0a1e..10095ae05826 100644 > > --- a/kernel/debug/kdb/kdb_bt.c > > +++ b/kernel/debug/kdb/kdb_bt.c > > @@ -10,6 +10,7 @@ > > */ > > > > #include <linux/ctype.h> > > +#include <linux/delay.h> > > #include <linux/string.h> > > #include <linux/kernel.h> > > #include <linux/sched/signal.h> > > @@ -22,20 +23,43 @@ > > static void kdb_show_stack(struct task_struct *p, void *addr) > > { > > int old_lvl = console_loglevel; > > + int time_left; > > + int cpu; > > + > > console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH; > > kdb_trap_printk++; > > - kdb_set_current_task(p); > > - if (addr) { > > - show_stack((struct task_struct *)p, addr); > > - } else if (kdb_current_regs) { > > -#ifdef CONFIG_X86 > > - show_stack(p, &kdb_current_regs->sp); > > -#else > > - show_stack(p, NULL); > > -#endif > > + > > + if (!addr && kdb_task_has_cpu(p)) { > > + cpu = kdb_process_cpu(p); > > + > > + if (cpu == raw_smp_processor_id()) { > > + dump_stack(); > > + goto exit; > > This goto is not for error recovery but looks like it is. Why can't we > use normal control flow here (extracting the remote stack dump logic > into a seperate function if the right margin is getting too close)? > > In fact to be honest a function call would be useful anyway since I'd > rather have all the resulting horror in a single file (debug_core.c).
Sure. Done.
> > + } > > + > > + /* > > + * In general architectures don't support dumping the stack > > + * of a "running" process that's not the current one so if > > + * we want to dump the stack of a running process that's not > > + * the master then we'll set a global letting the slave > > + * (which should be looping) know to dump its own stack. > > + */ > > + dbg_slave_dumpstack_cpu = cpu; > > + for (time_left = MSEC_PER_SEC; time_left; time_left--) { > > + udelay(1000); > > + if (dbg_slave_dumpstack_cpu == -1) > > + break; > > + } > > This timeout does not interact correctly with the pager (the timer does > not get reset when we sit in the pager loop waiting for user to tell us > to continue).
Nice catch, thanks. Probably the easiest thing to do is to get rid of this timeout but put in a check to make sure that the CPU has the "IN_SLAVE" flag set. This was important since you otherwise could get into this code path by doing "ps" to see what process was running on a non-rounded-up CPU and then "btp <pid>". v3 should handle this without the timeout.
-Doug
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