linux-kernel - Re: [PATCH v2 1/3] perf/core: Add a tracepoint for perf sampling

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Message-ID: <20160805014341.GB52225@ast-mbp.thefacebook.com>
Date:	Thu, 4 Aug 2016 18:43:43 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Brendan Gregg <bgregg@...flix.com>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
	Wang Nan <wangnan0@...wei.com>
Subject: Re: [PATCH v2 1/3] perf/core: Add a tracepoint for perf sampling

On Thu, Aug 04, 2016 at 04:28:53PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 03, 2016 at 11:57:05AM -0700, Brendan Gregg wrote:
> 
> > As for pmu tracepoints: if I were to instrument it (although I wasn't
> > planning to), I'd put a tracepoint in perf_event_overflow() called
> > "perf:perf_overflow", with the same arguments. That could then be used
> > for all PMU overflow events, without needing to add specific
> > tracepoints. 
> 
> Could we not teach BPF to replace event->overflow_handler and inject
> itself there?
> 
> We don't currently have nice interfaces for doing that, but it should be
> possible to do I think. We already have the indirect function call, so
> injecting ourself there has 0 overhead.

you're right. All makes sense. I guess I was too lazy to look into
how to do it properly. Adding a tracepoint looked like quick and
easy way to achieve the same.
As far as api goes probably existing IOC_SET_BPF ioctl will do too.
Currently overflow_handler is set at event alloc time. If we start
changing it on the fly with atomic xchg(), afaik things shouldn't
break, since each overflow_handler is run to completion and doesn't
change global state, right?