From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
Vince Weaver <vincent.weaver@maine.edu>,
linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Paul Turner <pjt@google.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: perf: multiple mmap of fd behavior on x86/ARM
Date: Mon, 14 Aug 2017 11:57:29 +0100 [thread overview]
Message-ID: <20170814105729.GA14337@arm.com> (raw)
In-Reply-To: <20170811145330.gtgx3ujmrn6izoka@hirez.programming.kicks-ass.net>
On Fri, Aug 11, 2017 at 04:53:30PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 11, 2017 at 12:06:39PM +0100, Mark Rutland wrote:
> > On Fri, Aug 11, 2017 at 12:52:52PM +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 11, 2017 at 11:01:27AM +0100, Mark Rutland wrote:
> > > > On Thu, Aug 10, 2017 at 02:48:52PM -0400, Vince Weaver wrote:
> > > > >
> > > > > So I was working on my perf_event_tests on ARM/ARM64 (the end goal was to
> > > > > get ARM64 rdpmc support working, but apparently those patches never made
> > > > > it upstream?)
> > > >
> > > > IIUC by 'rdpmc' you mean direct userspace counter access?
> > > >
> > > > Patches for that never made it upstream. Last I saw, there were no
> > > > patches in a suitable state for review.
> > > >
> > > > There are also difficulties (e.g. big.LITTLE systems where the number of
> > > > counters can differ across CPUs) which have yet to be solved.
> > >
> > > How would that be a problem? The API gives an explicit index to use with
> > > the 'rdpmc' instruction.
> >
> > It's a problem because access to unimplemented counters trap. So if a
> > task gets migrated from a CPU with N counters to one with N-1, accessing
> > counter N would be problematic.
> >
> > So we'd need to account for that somehow, in addition to the usual
> > sequence counter fun to verify the index was valid when the access was
> > performed.
>
> Aah, you need restartable-sequences :-)
Or, in the absence of those, I wouldn't mind only supporting this for
non-big/little platforms initially.
Will
next prev parent reply other threads:[~2017-08-14 10:57 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-10 18:48 perf: multiple mmap of fd behavior on x86/ARM Vince Weaver
2017-08-11 10:01 ` Mark Rutland
2017-08-11 10:52 ` Peter Zijlstra
2017-08-11 11:06 ` Mark Rutland
2017-08-11 14:53 ` Peter Zijlstra
2017-08-14 10:57 ` Will Deacon [this message]
2017-08-11 15:25 ` Vince Weaver
2017-08-11 16:23 ` Mark Rutland
2017-08-11 16:51 ` Vince Weaver
2017-08-11 17:09 ` Mark Rutland
2017-08-11 19:01 ` Vince Weaver
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