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Message-ID: <20091001085330.GC15345@elte.hu>
Date: Thu, 1 Oct 2009 10:53:30 +0200
From: Ingo Molnar <mingo@...e.hu>
To: "K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc: Arjan van de Ven <arjan@...radead.org>,
"Frank Ch. Eigler" <fche@...hat.com>, peterz@...radead.org,
linux-kernel@...r.kernel.org,
Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH] perf_core: provide a kernel-internal interface to
get to performance counters
* K.Prasad <prasad@...ux.vnet.ibm.com> wrote:
> On Thu, Oct 01, 2009 at 09:25:18AM +0200, Ingo Molnar wrote:
> >
> > * Arjan van de Ven <arjan@...radead.org> wrote:
> >
> > > On Sun, 27 Sep 2009 00:02:46 +0530
> > > "K.Prasad" <prasad@...ux.vnet.ibm.com> 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
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