[RFC][PATCH 5/8] perf/x86/intel: Optimize intel_get_excl_constraints()

[Peter Zijlstra]

Avoid the POPCNT by noting we can decrement the weight for each
cleared bit.

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 arch/x86/events/intel/core.c |   22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2838,7 +2838,7 @@ intel_get_excl_constraints(struct cpu_hw
        struct intel_excl_cntrs *excl_cntrs = cpuc->excl_cntrs;
        struct intel_excl_states *xlo;
        int tid = cpuc->excl_thread_id;
-       int is_excl, i;
+       int is_excl, i, w;
 
        /*
         * validating a group does not require
@@ -2894,36 +2894,40 @@ intel_get_excl_constraints(struct cpu_hw
         * SHARED   : sibling counter measuring non-exclusive event
         * UNUSED   : sibling counter unused
         */
+       w = c->weight;
        for_each_set_bit(i, c->idxmsk, X86_PMC_IDX_MAX) {
                /*
                 * exclusive event in sibling counter
                 * our corresponding counter cannot be used
                 * regardless of our event
                 */
-               if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE)
+               if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE) {
                        __clear_bit(i, c->idxmsk);
+                       w--;
+                       continue;
+               }
                /*
                 * if measuring an exclusive event, sibling
                 * measuring non-exclusive, then counter cannot
                 * be used
                 */
-               if (is_excl && xlo->state[i] == INTEL_EXCL_SHARED)
+               if (is_excl && xlo->state[i] == INTEL_EXCL_SHARED) {
                        __clear_bit(i, c->idxmsk);
+                       w--;
+                       continue;
+               }
        }
 
        /*
-        * recompute actual bit weight for scheduling algorithm
-        */
-       c->weight = hweight64(c->idxmsk64);
-
-       /*
         * if we return an empty mask, then switch
         * back to static empty constraint to avoid
         * the cost of freeing later on
         */
-       if (c->weight == 0)
+       if (!w)
                c = &emptyconstraint;
 
+       c->weight = w;
+
        return c;
 }