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

From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: "K.Prasad" <prasad@linux.vnet.ibm.com>,
	Arjan van de Ven <arjan@infradead.org>,
	"Frank Ch. Eigler" <fche@redhat.com>,
	linux-kernel@vger.kernel.org,
	Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [RFC PATCH] perf_core: provide a kernel-internal interface to get to performance counters
Date: Mon, 05 Oct 2009 09:53:30 +0200	[thread overview]
Message-ID: <1254729210.26976.15.camel@twins> (raw)
In-Reply-To: <20091001085330.GC15345@elte.hu>

On Thu, 2009-10-01 at 10:53 +0200, Ingo Molnar wrote:
> 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.

Non-trivial.

Something like this would imply a single output channel for all these
CPUs, and we've already seen that stuffing too many CPUs down one such
channel (using -M) leads to significant performance issues.

Therefore I would strongly argue to let the kernel interface be what it
is and solve this in a userspace library for those who care.

We really cannot sanely support an all-CPUs abstraction without running
into trouble.