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

[Waiman Long]

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.

Thanks,
Longman