linux-kernel - Re: perf: rdpmc bug when viewing all procs on remote cpu

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Date:   Fri, 18 Jan 2019 13:01:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vince Weaver <vincent.weaver@...ne.edu>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: rdpmc bug when viewing all procs on remote cpu

On Fri, Jan 11, 2019 at 04:52:22PM -0500, Vince Weaver wrote:
> On Thu, 10 Jan 2019, Vince Weaver wrote:
> 
> > On Thu, 10 Jan 2019, Vince Weaver wrote:
> > 
> > > On Thu, 10 Jan 2019, Vince Weaver wrote:
> > > 
> > > > However if you create an all-process attached to CPU event:
> > > > 	perf_event_open(attr, -1, X, -1, 0);
> > > > the mmap event index is set as if this were a valid event and so the rdpmc
> > > > succeeds even though it shouldn't (we're trying to read an event value
> > > > on a remote cpu with a local rdpmc).
> 
> so on further looking at the code, it doesn't appear that rdpmc events are 
> explicitly marked as unavailable in the attach-cpu or attach-pid case, 
> it's just by luck the check for PERF_EVENT_STATE_ACTIVE catches most of 
> the cases?
> 
> should an explicit check be added to zero out userpg->index in cases where 
> the event being measured is running on a different core?

And how would we konw? We don't know what CPU will be observing the
mmap().

RDPMC fundamentally only makes sense on 'self' (either task or CPU).