Fix performance regression on lmbench select benchmark
Linus Torvalds [Sun, 22 Jun 2008 19:23:15 +0000 (12:23 -0700)]
Christian Borntraeger reported that reinstating cond_resched() with
CONFIG_PREEMPT caused a performance regression on lmbench:

For example select file 500:
23 microseconds
32 microseconds

and that's really because we totally unnecessarily do the cond_resched()
in the innermost loop of select(), which is just silly.

This moves it out from the innermost loop (which only ever loops ove the
bits in a single "unsigned long" anyway), which makes the performance
regression go away.

Reported-and-tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/select.c

index 8dda969..da0e882 100644 (file)
@@ -249,7 +249,6 @@ int do_select(int n, fd_set_bits *fds, s64 *timeout)
                                                retval++;
                                        }
                                }
-                               cond_resched();
                        }
                        if (res_in)
                                *rinp = res_in;
@@ -257,6 +256,7 @@ int do_select(int n, fd_set_bits *fds, s64 *timeout)
                                *routp = res_out;
                        if (res_ex)
                                *rexp = res_ex;
+                       cond_resched();
                }
                wait = NULL;
                if (retval || !*timeout || signal_pending(current))