MDS/CDir: return as early as possible from CDir::should_split_fast()

All we want to know is whether we're above the `fast_limit`; if we're
above that, we don't need to know how many exactly.

By returning early instead of iterating over all entries, a lot of CPU
time can be saved.  In a microbenchmark where `fio` was used to create
thousands of files, the CPU usage of `CDir::should_split_fast()` went
from 6% to less than 1% in the `perf report`.

Signed-off-by: Max Kellermann <[email protected]>

Author: MaxKellermann

src/mds/CDir.cc

@@ -3811,10 +3811,13 @@ bool CDir::should_split_fast() const
     const CDentry *dn = p.second;
     if (!dn->get_projected_linkage()->is_null()) {
       effective_size++;
+
+      if (effective_size > fast_limit) [[unlikely]]
+	return true;
     }
   }
 
-  return effective_size > fast_limit;
+  return false;
 }
 
 bool CDir::should_merge() const