Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)

[Waiman Long <longman@redhat.com>]

On 2/9/22 14:17, Mathieu Desnoyers wrote:
> ----- On Feb 9, 2022, at 2:02 PM, Waiman Long longman@redhat.com wrote:
>
>> On 2/9/22 13:29, Mathieu Desnoyers wrote:
>>> ----- On Feb 9, 2022, at 1:19 PM, Waiman Long longman@redhat.com wrote:
>>>
>>>> On 2/9/22 04:09, Peter Zijlstra wrote:
>>>>> On Tue, Feb 08, 2022 at 10:41:56AM -0800, Namhyung Kim wrote:
>>>>>
>>>>>> Eventually I'm mostly interested in the contended locks only and I
>>>>>> want to reduce the overhead in the fast path.  By moving that, it'd be
>>>>>> easy to track contended locks with timing by using two tracepoints.
>>>>> So why not put in two new tracepoints and call it a day?
>>>>>
>>>>> Why muck about with all that lockdep stuff just to preserve the name
>>>>> (and in the process continue to blow up data structures etc..). This
>>>>> leaves distros in a bind, will they enable this config and provide
>>>>> tracepoints while bloating the data structures and destroying things
>>>>> like lockref (which relies on sizeof(spinlock_t)), or not provide this
>>>>> at all.
>>>>>
>>>>> Yes, the name is convenient, but it's just not worth it IMO. It makes
>>>>> the whole proposition too much of a trade-off.
>>>>>
>>>>> Would it not be possible to reconstruct enough useful information from
>>>>> the lock callsite?
>>>>>
>>>> I second that as I don't want to see the size of a spinlock exceeds 4
>>>> bytes in a production system.
>>>>
>>>> Instead of storing additional information (e.g. lock name) directly into
>>>> the lock itself. Maybe we can store it elsewhere and use the lock
>>>> address as the key to locate it in a hash table. We can certainly extend
>>>> the various lock init functions to do that. It will be trickier for
>>>> statically initialized locks, but we can probably find a way to do that too.
>>> If we go down that route, it would be nice if we can support a few different
>>> use-cases for various tracers out there.
>>>
>>> One use-case (a) requires the ability to query the lock name based on its
>>> address as key.
>>> For this a hash table is a good fit. This would allow tracers like ftrace to
>>> output lock names in its human-readable output which is formatted within the
>>> kernel.
>>>
>>> Another use-case (b) is to be able to "dump" the lock { name, address } tuples
>>> into the trace stream (we call this statedump events in lttng), and do the
>>> translation from address to name at post-processing. This simply requires
>>> that this information is available for iteration for both the core kernel
>>> and module locks, so the tracer can dump this information on trace start
>>> and module load.
>>>
>>> Use-case (b) is very similar to what is done for the kernel tracepoints. Based
>>> on this, implementing the init code that iterates on those sections and
>>> populates
>>> a hash table for use-case (a) should be easy enough.
>> Yes, that are good use cases for this type of functionality. I do need
>> to think about how to do it for statically initialized lock first.
> Tracepoints already solved that problem.
>
> Look at the macro DEFINE_TRACE_FN() in include/linux/tracepoint.h. You will notice that
> it statically defines a struct tracepoint in a separate section and a tracepoint_ptr_t
> in a __tracepoints_ptrs section.
>
> Then the other parts of the picture are in kernel/tracepoint.c:
>
> extern tracepoint_ptr_t __start___tracepoints_ptrs[];
> extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
>
> and kernel/module.c:find_module_sections()
>
> #ifdef CONFIG_TRACEPOINTS
>          mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
>                                               sizeof(*mod->tracepoints_ptrs),
>                                               &mod->num_tracepoints);
> #endif
>
> and the iteration code over kernel and modules in kernel/tracepoint.c.
>
> All you need in addition is in include/asm-generic/vmlinux.lds.h, we add
> to the DATA_DATA define an entry such as:
>
>          STRUCT_ALIGN();                                                 \
>          *(__tracepoints)                                                \
>
> and in RO_DATA:
>
>                  . = ALIGN(8);                                           \
>                  __start___tracepoints_ptrs = .;                         \
>                  KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \
>                  __stop___tracepoints_ptrs = .;
>
> AFAIU, if you do something similar for a structure that contains your relevant
> lock information, it should be straightforward to handle statically initialized
> locks.
>
> Thanks,
>
> Mathieu

Thanks for the suggestion.

Cheers,
Longman