Commit b5d9d734 authored Sep 08, 2009 by Mike Galbraith Committed by Ingo Molnar Sep 08, 2009

sched: Ensure that a child can't gain time over it's parent after fork()



A fork/exec load is usually "pass the baton", so the child
should never be placed behind the parent.  With START_DEBIT we
make room for the new task, but with child_runs_first, that
room comes out of the _parent's_ hide. There's nothing to say
that the parent wasn't ahead of min_vruntime at fork() time,
which means that the "baton carrier", who is essentially the
parent in drag, can gain time and increase scheduling latencies
for waiters.

With NEW_FAIR_SLEEPERS + START_DEBIT + child_runs_first
enabled, we essentially pass the sleeper fairness off to the
child, which is fine, but if we don't base placement on the
parent's updated vruntime, we can end up compounding latency
woes if the child itself then does fork/exec.  The debit
incurred at fork doesn't hurt the parent who is then going to
sleep and maybe exit, but the child who acquires the error
harms all comers.

This improves latencies of make -j<n> kernel build workloads.

Reported-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent a8fae3ec

kernel/sched_fair.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -728,10 +728,10 @@ place_entity(struct cfs_rq cfs_rq, struct sched_entity se, int initial)

		vruntime -= thresh;
		}
		}

		/* ensure we never gain time by being placed backwards. */
		vruntime = max_vruntime(se->vruntime, vruntime);
		}

		se->vruntime = vruntime;
		}
		@@ -1756,6 +1756,8 @@ static void task_new_fair(struct rq rq, struct task_struct p)
		sched_info_queued(p);

		update_curr(cfs_rq);
		if (curr)
		se->vruntime = curr->vruntime;
		place_entity(cfs_rq, se, 1);

		/* 'curr' will be NULL if the child belongs to a different group */

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