Messages in this thread Patch in this message | | | Subject | Re: [PATCH 6/8] sched: avg_vruntime | From | Peter Zijlstra <> | Date | Wed, 29 Oct 2008 16:48:34 +0100 |
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On Fri, 2008-10-24 at 11:06 +0200, Peter Zijlstra wrote: > plain text document attachment (sched-avg-vruntime.patch) > Renicing requires scaling the lag. Therefore we need a way to compute the it. > Lag is defined as the difference between the service time received from the > ideal model and the actual scheduler. > > The defining property of a fair scheduler is that the sum of all lags is zero; > which can be seen is trivially true for the ideal case, as all lags are zero. > > Therefore, the average of all virtual runtimes will be the point of zero lag. > > We cannot prove fairness for CFS due to sleeper fairness (without it we can). > However since we can observe it does converge to fairness in stable operation, > we can say the zero lag point converges to the average. > > We can't just take the average of vruntime - as it will use the full range > of its u64 and will wrap around. Instead we'll use the average of > (vruntime - min_vruntime) > > \Sum_{i}^{n} 1/n (v_{i} - v) = 1/n (\Sum_{i}^{n} v_{i}) - vn > > By factoring out the 1/n (never storing that) we avoid rounding, which > would bring an accumulating error.
Hi Fabio,
you were right, this is wrong.
How about this..
The fluid model, would for each task t_i, generate an execution time e_i
de_i = w_i / w_sum * dt
However, any real scheduler will be imperfect and have an error eps_i
dE_i = de_i + eps_i,
But due to only dt actual time having past we can state that
\Sum_i dE_i = dt, therefore \Sum_i eps_i = 0.
This will be reflected in a virtual runtime skew of
dv_i = eps_i / w_i
If we now wish to obtain the zero lag point, there were all tasks would be in the fluid model, we get
eps_i = dv_i * w_i, which yields: \Sum dv_i * w_i = 0
IOW avg(v_i*w_i) = v_fluid
1/n \Sum_i v_i*w_i, [v_i -> v_i-x] -> 1/n \sum_i (v_i-x)*w_i = 1/n \Sum v_i*w_i - \Sum x*w_i = 1/n \Sum v_i*w_i - x \Sum w_i
which in turn would yield a patch like below..
I'll also try and quantify the error and effect of using min_vruntime as zero lag point as Ingo suggested.
--- Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c 2008-10-29 16:43:16.000000000 +0100 +++ linux-2.6/kernel/sched.c 2008-10-29 16:43:27.000000000 +0100 @@ -384,6 +384,10 @@ struct cfs_rq { struct load_weight load; unsigned long nr_running; + long nr_queued; + long avg_load; + s64 avg_vruntime; + u64 exec_clock; u64 min_vruntime; Index: linux-2.6/kernel/sched_debug.c =================================================================== --- linux-2.6.orig/kernel/sched_debug.c 2008-10-29 16:43:04.000000000 +0100 +++ linux-2.6/kernel/sched_debug.c 2008-10-29 16:43:37.000000000 +0100 @@ -161,6 +161,9 @@ void print_cfs_rq(struct seq_file *m, in SPLIT_NS(spread0)); SEQ_printf(m, " .%-30s: %ld\n", "nr_running", cfs_rq->nr_running); SEQ_printf(m, " .%-30s: %ld\n", "load", cfs_rq->load.weight); + SEQ_printf(m, " .%-30s: %Ld.%06ld\n", "avg_vruntime", + SPLIT_NS(avg_vruntime(cfs_rq))); + #ifdef CONFIG_SCHEDSTATS #define P(n) SEQ_printf(m, " .%-30s: %d\n", #n, rq->n); Index: linux-2.6/kernel/sched_fair.c =================================================================== --- linux-2.6.orig/kernel/sched_fair.c 2008-10-29 16:43:17.000000000 +0100 +++ linux-2.6/kernel/sched_fair.c 2008-10-29 16:46:41.000000000 +0100 @@ -271,6 +271,60 @@ static inline s64 entity_key(struct cfs_ return se->vruntime - cfs_rq->min_vruntime; } +static void +avg_vruntime_add(struct cfs_rq *cfs_rq, struct sched_entity *se) +{ + s64 key = entity_key(cfs_rq, se); + cfs_rq->avg_load += se->load.weight; + cfs_rq->avg_vruntime += key * se->load.weight; + cfs_rq->nr_queued++; +} + +static void +avg_vruntime_sub(struct cfs_rq *cfs_rq, struct sched_entity *se) +{ + s64 key = entity_key(cfs_rq, se); + cfs_rq->avg_load -= se->load.weight; + cfs_rq->avg_vruntime -= key * se->load.weight; + cfs_rq->nr_queued--; +} + +static inline +void avg_vruntime_update(struct cfs_rq *cfs_rq, s64 delta) +{ + cfs_rq->avg_vruntime -= cfs_rq->nr_queued * cfs_rq->avg_load * delta; +} + +static u64 avg_vruntime(struct cfs_rq *cfs_rq) +{ + s64 avg = cfs_rq->avg_vruntime; + long nr_queued = cfs_rq->nr_queued; + + if (cfs_rq->curr) { + nr_queued++; + avg += entity_key(cfs_rq, cfs_rq->curr) * cfs_rq->curr->load.weight; + } + + avg >>= NICE_0_SHIFT; + + if (nr_queued) + avg = div_s64(avg, nr_queued); + + return cfs_rq->min_vruntime + avg; +} + +static void __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime) +{ + /* + * open coded max_vruntime() to allow updating avg_vruntime + */ + s64 delta = (s64)(vruntime - cfs_rq->min_vruntime); + if (delta > 0) { + avg_vruntime_update(cfs_rq, delta); + cfs_rq->min_vruntime = vruntime; + } +} + static void update_min_vruntime(struct cfs_rq *cfs_rq) { u64 vruntime = cfs_rq->min_vruntime; @@ -289,7 +343,7 @@ static void update_min_vruntime(struct c vruntime = min_vruntime(vruntime, se->vruntime); } - cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime); + __update_min_vruntime(cfs_rq, vruntime); } /* @@ -303,6 +357,8 @@ static void __enqueue_entity(struct cfs_ s64 key = entity_key(cfs_rq, se); int leftmost = 1; + avg_vruntime_add(cfs_rq, se); + /* * Find the right place in the rbtree: */ @@ -345,6 +401,7 @@ static void __dequeue_entity(struct cfs_ cfs_rq->next = NULL; rb_erase(&se->run_node, &cfs_rq->tasks_timeline); + avg_vruntime_sub(cfs_rq, se); } static inline struct rb_node *first_fair(struct cfs_rq *cfs_rq)
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