On 22.02.21 22:18, Song Bao Hua (Barry Song) wrote: > > >> -----Original Message----- >> From: Kieran Bingham [mailto:kieran.bingham@xxxxxxxxxxxxxxxx] >> Sent: Tuesday, February 23, 2021 12:06 AM >> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; corbet@xxxxxxx; >> linux-doc@xxxxxxxxxxxxxxx; jan.kiszka@xxxxxxxxxxx >> Cc: linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx >> Subject: Re: [PATCH] scripts/gdb: document lx_current is only supported by x86 >> >> Hi Barry >> >> On 21/02/2021 21:35, Barry Song wrote: >>> lx_current depends on the per_cpu current_task which exists on x86 only: >>> >>> arch$ git grep current_task | grep -i per_cpu >>> x86/include/asm/current.h:DECLARE_PER_CPU(struct task_struct *, >> current_task); >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, current_task) >> ____cacheline_aligned = >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task); >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, current_task) >> = &init_task; >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task); >>> x86/kernel/smpboot.c: per_cpu(current_task, cpu) = idle; >>> >>> On other architectures, lx_current() will lead to a python exception: >>> (gdb) p $lx_current().pid >>> Python Exception <class 'gdb.error'> No symbol "current_task" in current >> context.: >>> Error occurred in Python: No symbol "current_task" in current context. >>> >>> To avoid more people struggling and wasting time in other architectures, >>> document it. >>> >>> Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx> >>> --- >>> Documentation/dev-tools/gdb-kernel-debugging.rst | 2 +- >>> scripts/gdb/linux/cpus.py | 10 ++++++++-- >>> 2 files changed, 9 insertions(+), 3 deletions(-) >>> >>> diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst >> b/Documentation/dev-tools/gdb-kernel-debugging.rst >>> index 4756f6b3a04e..1586901b683c 100644 >>> --- a/Documentation/dev-tools/gdb-kernel-debugging.rst >>> +++ b/Documentation/dev-tools/gdb-kernel-debugging.rst >>> @@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb helpers >>> [ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] >> reserved >>> .... >>> >>> -- Examine fields of the current task struct:: >>> +- Examine fields of the current task struct(supported by x86 only):: >>> >>> (gdb) p $lx_current().pid >>> $1 = 4998 >>> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py >>> index 008e62f3190d..f382762509d3 100644 >>> --- a/scripts/gdb/linux/cpus.py >>> +++ b/scripts/gdb/linux/cpus.py >>> @@ -156,6 +156,13 @@ Note that VAR has to be quoted as string.""" >>> >>> PerCpu() >>> >>> +def get_current_task(cpu): >>> + if utils.is_target_arch("x86"): >>> + var_ptr = gdb.parse_and_eval("¤t_task") >>> + return per_cpu(var_ptr, cpu).dereference() >>> + else: >>> + raise gdb.GdbError("Sorry, obtaining the current task is not yet " >>> + "supported with this arch") >> >> I've wondered in the past how we should handle the architecture specific >> layers. >> >> Perhaps we need to have an interface of functionality to implement on >> each architecture so that we can create a per-arch set of helpers. >> >> or break it up into arch specific subdirs at least... >> >> >>> class LxCurrentFunc(gdb.Function): >>> """Return current task. >>> @@ -167,8 +174,7 @@ number. If CPU is omitted, the CPU of the current context >> is used.""" >>> super(LxCurrentFunc, self).__init__("lx_current") >>> >>> def invoke(self, cpu=-1): >>> - var_ptr = gdb.parse_and_eval("¤t_task") >>> - return per_cpu(var_ptr, cpu).dereference() >>> + return get_current_task(cpu) >>> >> >> And then perhaps we simply shouldn't even expose commands which can not >> be supported on those architectures? > > I feel it is better to tell users this function is not supported on its arch > than simply hiding the function. > > If we hide it, users still have many chances to try it as they have got > information of lx_current from google or somewhere. > They will try, if it turns out the lx_current is not in the list and an > error like "invalid data type for function to be called", they will > probably suspect their gdb/python environment is not set up correctly, > and continue to waste time in checking their environment. > Finally they figure out this function is not supported by its arch so it is > not exposed. But they have wasted a couple of hours before knowing that. > > It seems it is more friendly to directly tell users this is not supported > on its arch explicitly and clearly than reporting a "invalid data type > for function to be called. > >> >> Is it easy to disable this command if it's not supportable on the >> architecture? >> > > TBH, I'm not a python expert. I don't know how to do that in an elegant > way :-) on the other hand, it seems lx_current isn’t a command like > lx-dmesg. Lx_current is actually similar with lx_per_cpu, we use gdb's > print(p) command to show its content. > >> Presumably you are working on non-x86, have you investigated adding this >> support for your architecture (arm/arm64?)? > > Yes. I've thought about it. But It would be quite trivial to bring up > this function on arm64. > > arch/arm64/include/asm/current.h: > static __always_inline struct task_struct *get_current(void) > { > unsigned long sp_el0; > > asm ("mrs %0, sp_el0" : "=r" (sp_el0)); > > return (struct task_struct *)sp_el0; > } > > We have to read a special register named sp_el0 and convert it to > task_struct while we are running in kernel mode. In gdb I can do > it by: > (gdb)p/x $SP_EL0 > $20 = 0xffffffc011492400 > (gdb)p ((struct task_struct *0xffffffc011492400))->pid > $21 = 0 > > What is more complex is that if we are running in user mode(EL0), this > register doesn't describe current task any more. so we have to > differentiate the modes of processor and make sure it only returns > current task while we are running in EL1(processor's kernel mode). Is all information needed for this available via gdb? > >> >> The fact you have run the command implies it would be useful for you ? >> > > Yes. I think it is a common requirement to get current task. lx_current > convenience function can help everyone. Since there is a document saying > this command exists, everyone using scripts/gdb would like to try it > I guess. > > The simplest way would be adding current_task per_cpu variable for other > arch, but I believe hardly arch maintainers will accept it as its only > benefit is bringing up the lx_current. So 99.9% no maintainer wants it. > > Thus, for the time being, I moved to just stop people from wasting time > like what I had done with a couple of hours. > I agree with the warning, also as potential motivation to add support for other archs. Jan -- Siemens AG, T RDA IOT Corporate Competence Center Embedded Linux
