Re: [RFC PATCH] perf_core: provide a kernel-internal interface toget to performance counters

[Frederic Weisbecker]

On Thu, Oct 01, 2009 at 10:53:30AM +0200, Ingo Molnar wrote:
> 
> * K.Prasad <prasad@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, Oct 01, 2009 at 09:25:18AM +0200, Ingo Molnar wrote:
> > > 
> > > * Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
> > > 
> > > > On Sun, 27 Sep 2009 00:02:46 +0530
> > > > "K.Prasad" <prasad@xxxxxxxxxxxxxxxxxx> wrote:
> > > > 
> > > > > On Sat, Sep 26, 2009 at 12:03:28PM -0400, Frank Ch. Eigler wrote:
> > > > 
> > > > > > For what it's worth, this sort of thing also looks useful from 
> > > > > > systemtap's point of view.
> > > > > 
> > > > > Wouldn't SystemTap be another user that desires support for 
> > > > > multiple/all CPU perf-counters (apart from hw-breakpoints as a 
> > > > > potential user)? As Arjan pointed out, perf's present design would 
> > > > > support only a per-CPU or per-task counter; not both.
> > > > 
> > > > I'm sorry but I think I am missing your point. "all cpu counters" 
> > > > would be one small helper wrapper away, a helper I'm sure the 
> > > > SystemTap people are happy to submit as part of their patch series 
> > > > when they submit SystemTap to the kernel.
> > > 
> > > Yes, and Frederic wrote that wrapper already for the hw-breakpoints 
> > > patches. It's a non-issue and does not affect the design - we can always 
> > > gang up an array of per cpu perf events, it's a straightforward use of 
> > > the existing design.
> > > 
> > 
> > Such a design (iteratively invoking a per-CPU perf event for all 
> > desired CPUs) isn't without issues, some of which are noted here: 
> > (apart from http://lkml.org/lkml/2009/9/14/298).
> > 
> > - It breaks the abstraction that a user of the exported interfaces would
> >   enjoy w.r.t. having all CPU (or a cpumask of CPU) breakpoints.
> 
> CPU offlining/onlining support would be interesting to add.
> 
> > - (Un)Availability of debug registers on every requested CPU is not
> >   known until request for that CPU fails. A failed request should be 
> >   followed by a rollback of the partially successful requests.
> 
> Yes.
> 
> > - Any breakpoint exceptions generated due to partially successful
> >   requests (before a failed request is encountered) must be treated as 
> >   'stray' and be ignored (by the end-user? or the wrapper code?).
> 
> Such inatomicity is inherent in using more than one CPU and a disjoint 
> set of hw-breakpoints. If the calling code cares then callbacks 
> triggering while the registration has not returned yet can be ignored.
> 
> > - Any CPUs that become online eventually have to be trapped and
> >   populated with the appropriate debug register value (not something 
> >   that the end-user of breakpoints should be bothered with).
> > 
> > - Modifying the characteristics of a kernel breakpoint (including the
> >   valid CPUs) will be equally painful.
> > 
> > - Races between the requests (also leading to temporary failure of
> >   all CPU requests) presenting an unclear picture about free debug
> >   registers (making it difficult to predict the need for a retry).
> > 
> > So we either have a perf event infrastructure that is cognisant of 
> > many/all CPU counters, or make perf as a user of hw-breakpoints layer 
> > which already handles such requests in a deft manner (through 
> > appropriate book-keeping).
> 
> Given that these are all still in the add-on category not affecting the 
> design, while the problems solved by perf events are definitely in the 
> non-trivial category, i'd suggest you extend perf events with a 'system 
> wide' event abstraction, which:
> 
>  - Enumerates such registered events (via a list)
> 
>  - Adds a CPU hotplug handler (which clones those events over to a new
>    CPU and directs it back to the ring-buffer of the existing event(s)
>    [if any])
> 
>  - Plus a state field that allows the filtering out of stray/premature
>    events.
> 
> Such an add-on layer/abstraction would sure be useful in other cases as 
> well. It might make sense to expose it to user-space and make perf top 
> use it by default.
> 
> Thanks,
> 
> 	Ingo


Can't we instead modify the perf events to be able to
run on multiple contexts?

We could change struct perf_event::ctx into a list of
context and then attach it to several cpu contexts.

The perf event struct have been designed to run on only one context
so its structure and handling does not deal with races due to
concurrent uses I guess. But at a first glance, few things would
need to be modified to handle that, and at a low cost.

There might be bad corner cases I forget though...

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