From b5d9d734a53e0204aab0089079cbde2a1285a38f Mon Sep 17 00:00:00 2001 From: Mike Galbraith Date: Tue, 8 Sep 2009 11:12:28 +0200 Subject: 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 kernel build workloads. Reported-by: Jens Axboe Signed-off-by: Mike Galbraith Acked-by: Peter Zijlstra LKML-Reference: Signed-off-by: Ingo Molnar --- kernel/sched_fair.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index cc97ea498f24..e386e5dfcdae 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -728,11 +728,11 @@ 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); } + /* 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 */ -- cgit v1.2.2