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Message-ID: <20150113093603.GK23965@worktop.programming.kicks-ass.net>
Date: Tue, 13 Jan 2015 10:36:03 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vince Weaver <vincent.weaver@...ne.edu>
Cc: linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35
On Fri, Jan 09, 2015 at 11:26:54AM -0500, Vince Weaver wrote:
>
> On Fri, 9 Jan 2015, Peter Zijlstra wrote:
> > > So is this worth fixing seeing as apparently no one uses this feature?
> >
> > I think there's a fair argument for removing it, Ingo, Acme?
>
> could the functionality be replaced with a subsequent call to
> ioctl(PERF_EVENT_IOC_SET_OUTPUT)
Yes.
> Although I suppose there's a possibility for losing a small amount of data
> or some other reason that PERF_FLAG_FD_OUTPUT was introduced in the first
> place.
That's a natural race without solution. That is, because there is no
serialization between the sys_perf_event_open() and any possible acts of
data generation, we can't even talk about loosing data.
Even if it were all in a single syscall, there is no saying how long
that syscall would take to complete -- imagine its stuck on memory
allocation or whatnot. Similarly you could have issued the syscall a wee
bit later or whatnot.
> In addition, if we remove PERF_FLAG_FD_OUTPUT would there then be any
> reason to keep PERF_FLAG_FD_NO_GROUP around?
Good point, I think there was another potential user of that proposed
recently, but I can't quite remember where or what. Let me try and go
find that.
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