linux-kernel - Re: [PATCH 2/5] perf core: Set event's default overflow

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Message-ID: <20160323191359.GL6356@twins.programming.kicks-ass.net>
Date:	Wed, 23 Mar 2016 20:13:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Wang Nan <wangnan0@...wei.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, He Kuang <hekuang@...wei.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Zefan Li <lizefan@...wei.com>, pi3orama@....com
Subject: Re: [PATCH 2/5] perf core: Set event's default overflow_handler

On Wed, Mar 23, 2016 at 06:13:49PM +0000, Will Deacon wrote:

> > Will, why does it matter what the overflow handler is for this stuff?
> 
> Because ptrace registers an overflow handler for raising a SIGTRAP and
> ptrace users (e.g. GDB) expect to handle the single-stepping themselves.
> Perf, on the other hand, will livelock if the kernel doesn't do the
> stepping.

Ah makes sense.

> FWIW, I hate this whole thing. The only users of the perf side just seem
> to be people running whacky test cases and then pointing out the places
> where we're not identical to x86 :(

Yeah, its not my favourite part either. The whole breakpoint thing
doesn't quite behave like a regular PMU so its all a bit 'special'.