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

[Stephane Eranian]

On Fri, May 22, 2015 at 6:36 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.
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