Fix performance regression on lmbench select benchmark
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>
diff --git a/fs/select.c b/fs/select.c
index 8dda969..da0e882 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -249,7 +249,6 @@
retval++;
}
}
- cond_resched();
}
if (res_in)
*rinp = res_in;
@@ -257,6 +256,7 @@
*routp = res_out;
if (res_ex)
*rexp = res_ex;
+ cond_resched();
}
wait = NULL;
if (retval || !*timeout || signal_pending(current))