linux-kernel - Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint

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Date:	Fri, 22 May 2015 15:48:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter
 constraint

On Fri, May 22, 2015 at 06:40:49AM -0700, Stephane Eranian wrote:
> On Fri, May 22, 2015 at 6:36 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, May 22, 2015 at 06:29:47AM -0700, Stephane Eranian wrote:
> >> On Fri, May 22, 2015 at 6:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >> > On Fri, May 22, 2015 at 06:07:00AM -0700, Stephane Eranian wrote:
> >> >>
> >> >> One other thing I noticed is that the --n_excl needs to be protected by the
> >> >> excl_cntrs->lock in put_excl_constraints().
> >> >
> >> > Nah, its strictly per cpu.
> >>
> >> No. the excl_cntrs struct is pointed to by cpuc but it is shared between the
> >> sibling HT. Otherwise this would not work!
> >
> > n_excl is per cpuc, see the trickery with has_exclusive vs
> > exclusive_present on how I avoid the lock.
> 
> Yes, but I believe  you create a store forward penalty with this.
> You store 16bits and you load 32 bits on the same cache line.

The store and load are fairly well spaced -- the entire scheduling fast
path is in between.

And such a penalty is still cheap compared to locking, no?
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