* Tejun Heo [off-list ref] wrote:

Hello, Peter, Ingo.

11/26/2009 09:40 PM, Peter Zijlstra wrote:

quoted

CALLBACK_EVENT() would be my preferred name, and shouldn't live anywhere
near the regular tracing bits, the tracing bits could simply add another
callback in it when enabled.

I haven't looked at the mm code but if the scheduler callback 
requirement isn't gonna explode big time soon and we know which 
functions are the candidate callbacks at build time, I think this can 
be done pretty efficiently with an ulong enable mask per task and 
fixed function dispatch such that no callback case just goes through 
one likely() conditional test at the tracing point and callback cases 
are dispatched using conditional direct jump.

Yes - and that's what the tracepoints infrastructure is about.

Btw., longer term it will be faster than a mask check and a 
default-untaken conditional: there's ongoign work to offer runtime 
instruction patching features for tracing callbacks. There's the jump 
patching optimization and also the immediate values patching 
optimization.

We've got old-style notifiers for regular callbacks, we've got new-style 
tracepoints which are callbacks and event source descriptors - and what 
i'm asking for is to have _one_ callback mechanism, and to use that in 
the scheduler. 5 callbacks using 3 different facilities is excessive - 
i'd like to see just two callbacks using one facility.

	Ingo