Professional Linux Kernel Architecture 2.6.3 section 疑问
在Professional Linux Kernel Architecture 2.6.3 section 中作者对CFS 的place_entity 中sched_entity 的vruntime 更新提到:
However, if the sleeper has accumulated a large unfairness as indicated by a large se_vruntime value, the kernel must honor this. If se->vruntime is larger than the previously computed difference, it is kept as the vruntime of the process, which leads to a leftward placement on the red-black tree — recall that large vruntime values are good to schedule early!
这个就奇怪了，明明vruntime 越大，对应在rb tree 中的位置就越右，怎么还越左，然后越早scheduler.
再反过来看place_entity 这段代码: version linux kernel 3.4.10 in kernel/sched/fair.c
static voidplace_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial){u64 vruntime = cfs_rq->min_vruntime;/* * The 'current' period is already promised to the current tasks, * however the extra weight of the new task will slow them down a * little, place the new task so that it fits in the slot that * stays open at the end. */if (initial && sched_feat(START_DEBIT))vruntime += sched_vslice(cfs_rq, se);/* sleeps up to a single latency don't count. */if (!initial) {unsigned long thresh = sysctl_sched_latency;/* * Halve their sleep time's effect, to allow * for a gentler effect of sleepers: */if (sched_feat(GENTLE_FAIR_SLEEPERS))thresh >>= 1;vruntime -= thresh;}/* ensure we never gain time by being placed backwards. */vruntime = max_vruntime(se->vruntime, vruntime);se->vruntime = vruntime;}?
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/* ensure we never gain time by being placed backwards. */
vruntime = max_vruntime(se->vruntime, vruntime);
作者当时的用意应当是防止vruntime 出现倒流的情况； 反过来，我们考察两种case.
1. process 短暂sleep ，这时se->vruntime 一般比min_vruntime 要大，选择se->vruntime 无可厚非。
2. process 长时间sleep, 这时se->vruntime 已经要比min_vruntime 要小很多，通过在min_vruntime 削掉sysctl_sched_latency 可保证插入rb tree 时，位置越往左，从而这个进程即越早被sched 到。 这可让长时间sleep 的process 尽早执行，也算一种奖励把。
至于书上写到的，真的怪怪的。
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