Re: [PATCH] mm: cache largest vma

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 05, 2013 at 09:24:51AM +0100, Ingo Molnar wrote:

SNIP

> > 
> > Yeah that would be a nice interface. Speaking about that, it would be nice to get your input
> > on the proposed interface for toggle events.
> > 
> > It's still in an RFC state, although it's getting quite elaborated, and I believe we haven't
> > yet found a real direction to take for the tooling interface IIRC. For example the perf record
> > cmdline used to state toggle events based contexts was one of the parts we were not that confident about.
> > And we really don't want to take a wrong direction for that as it's going to be complicated
> > to handle in any case.
> > 
> > See this thread:
> > https://lwn.net/Articles/568602/
> 
> At the risk of hijacking this discussion, here's my take on triggers:
> 
> I think the primary interface should be to allow the disabling/enabling of 
> a specific event from other events.
> 
> From user-space it would be fd driven: add a perf attribute to allow a 
> specific event to set the state of another event if it triggers. The 
> 'other event' would be an fd, similar to how group events are specified.
> 
> An 'off' trigger sets the state to 0 (disabled).
> An 'on' trigger sets the state to 1 (enabled).
> 
> Using such a facility the measurement of deltas would need 3 events:
> 
>  - fd1: a cycles event that is created disabled
> 
>  - fd2: a kprobes event at the 'start' RIP, set to counting only,
>         connected to fd1, setting state to '1'
> 
>  - fd3: a kprobes event at the 'stop' RIP, set to counting only,
>         connected to fd1, setting state to '0'.
> 
> This way every time the (fd2) start-RIP kprobes event executes, the 
> trigger code sees that it's supposed to enable the (fd1) cycles event. 
> Every time the (fd3) stop-RIP kprobes event executes, the trigger code 
> sees that it's set to disable the (fd1) cycles event.

that's more or less how the current code works,
you can check this wiki for details:
https://perf.wiki.kernel.org/index.php/Jolsa_Features_Togle_Event

> 
> Instead of 'cycles event', it could count instructions, or pagefaults, or 
> cachemisses.

we made it general for any kind of event

> 
> ( If the (fd1) cycles event is a sampling event then this would allow nice 
>   things like the profiling of individual functions within the context of 
>   a specific system call, driven by triggers. )
> 
> In theory we could allow self-referential triggers as well: the first 
> execution of the trigger would disable itself. If the trigger state is not 
> on/off but a counter then this would allow 'take 100 samples then shut 
> off' type of functionality as well.

ok, there's something similar in ftrace and we already
discussed this for perf.. I'll check

> 
> But success primarily depends on how useful the tooling UI turns out to 
> be: create a nice Slang or GTK UI for kprobes and triggers, and/or turn it 
> into a really intuitive command line UI, and people will use it.
> 
> I think annotated assembly/source output is a really nice match for 
> triggers and kprobes, so I'd suggest the Slang TUI route ...

yep, current toggling command line UI is not much user friendly

but perhaps we should leave it there (because it seems it wont
get much better anyway) and focus more on Slang UI as the
target one..

CCing Arnaldo ;-)

thanks,
jirka