Messages in this thread | | | Date | Fri, 22 May 2015 15:48:11 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint |
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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@infradead.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@infradead.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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