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Commit 55d85384 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 481c5346

fs/select.c

+1 −1
Original line number Diff line number Diff line
@@ -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))