Try using the last Deferred reader slot first

Summary:
When trying to find an empty deferred reader slot, getting the current CPU can
take quite a few cycles, e.g. >1% CPU on SMC (https://fburl.com/434646643).

Let's track the last slot used by this thread and try that slot first before reading
CPU id and doing the search.

u-benchmark results seem to be improving generally (though a bit noisy and not
sure how much to trust). Results w/ this diff on left side:
P56648675

Reviewed By: nbronson

Differential Revision: D3857793

fbshipit-source-id: 8b1c005362c82e748a663100f889b0b99dc257fe

folly/SharedMutex.h

@@ -723,6 +723,10 @@ class SharedMutexImpl {
   // This is the starting location for Token-less unlock_shared().
   static FOLLY_SHAREDMUTEX_TLS uint32_t tls_lastTokenlessSlot;
 
+  // Last deferred reader slot used.
+  static FOLLY_SHAREDMUTEX_TLS uint32_t tls_lastDeferredReaderSlot;
+
+
   // Only indexes divisible by kDeferredSeparationFactor are used.
   // If any of those elements points to a SharedMutexImpl, then it
   // should be considered that there is a shared lock on that instance.
@@ -1343,6 +1347,15 @@ FOLLY_SHAREDMUTEX_TLS uint32_t
     SharedMutexImpl<ReaderPriority, Tag_, Atom, BlockImmediately>::
         tls_lastTokenlessSlot = 0;
 
+template <
+    bool ReaderPriority,
+    typename Tag_,
+    template <typename> class Atom,
+    bool BlockImmediately>
+FOLLY_SHAREDMUTEX_TLS uint32_t
+    SharedMutexImpl<ReaderPriority, Tag_, Atom, BlockImmediately>::
+        tls_lastDeferredReaderSlot = 0;
+
 template <
     bool ReaderPriority,
     typename Tag_,
@@ -1385,6 +1398,10 @@ bool SharedMutexImpl<ReaderPriority, Tag_, Atom, BlockImmediately>::
         (state & kHasS) >= (kNumSharedToStartDeferring - 1) * kIncrHasS;
     bool drainInProgress = ReaderPriority && (state & kBegunE) != 0;
     if (canAlreadyDefer || (aboveDeferThreshold && !drainInProgress)) {
+      /* Try using the most recent slot first. */
+      slot = tls_lastDeferredReaderSlot;
+      slotValue = deferredReader(slot)->load(std::memory_order_relaxed);
+      if (slotValue != 0) {
         // starting point for our empty-slot search, can change after
         // calling waitForZeroBits
         uint32_t bestSlot =
@@ -1399,10 +1416,12 @@ bool SharedMutexImpl<ReaderPriority, Tag_, Atom, BlockImmediately>::
           slotValue = deferredReader(slot)->load(std::memory_order_relaxed);
           if (slotValue == 0) {
             // found empty slot
+            tls_lastDeferredReaderSlot = slot;
             break;
           }
         }
       }
+    }
 
     if (slotValue != 0) {
       // not yet deferred, or no empty slots