linux-kernel - Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

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Date:	Tue, 5 May 2015 19:00:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"mingo@...nel.org" <mingo@...nel.org>,
	"acme@...radead.org" <acme@...radead.org>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

On Tue, May 05, 2015 at 04:36:51PM +0000, Liang, Kan wrote:
> > > RECORDs are generic, and should live in the core code.
> > >
> > > Also, you should introduce this RECORD in a separate patch.
> > 
> > On that, this is lacking a RECORD definition in
> > include/uapi/linux/perf_event.h:perf_event_type
> 
> The PERF_RECORD_LOST already defined in  perf_event_type.
> Are you suggesting to add a new dedicated RECORD type, like PERF_RECORD_COLLISION?

Yes, this should be a new RECORD, LOST_SAMPLES maybe.

The thing is, LOST is about the ring-buffer running out of space, this
is very much not the case here. Reusing it like this creates the
situation where userspace cannot tell what happened, and that is a very
bad thing indeed.

What we want to convey is that we dropped/lost a (number of) sample(s).

So the objection against the RECORD_COLLISIONS name is that it names the
reason we did something, but not the something we did.
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