On 2/9/22 14:17, Mathieu Desnoyers wrote:
----- On Feb 9, 2022, at 2:02 PM, Waiman Long longman@xxxxxxxxxx wrote:
On 2/9/22 13:29, Mathieu Desnoyers wrote:
----- On Feb 9, 2022, at 1:19 PM, Waiman Long longman@xxxxxxxxxx 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
