Messages in this thread Patch in this message | | | Subject | Re: questions on calc_delta_mine() in sched.c | From | Peter Zijlstra <> | Date | Fri, 02 May 2008 15:10:07 +0200 |
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On Fri, 2008-05-02 at 14:30 +0200, Peter Zijlstra wrote: > On Fri, 2008-05-02 at 14:14 +0200, Peter Zijlstra wrote: > > > And, no sadly I have no ideas on how to get rid of it ;-/ > > Well, not quite true - I've once thought about a small cache of recently > computed inverse values. Esp for say a ping-pong scenario you end up > re-computing basically the same values over and over again. > > Maybe something like this (fully untested):
This one builds and... boots
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> --- kernel/sched.c | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-)
Index: linux-2.6-2/kernel/sched.c =================================================================== --- linux-2.6-2.orig/kernel/sched.c +++ linux-2.6-2/kernel/sched.c @@ -537,6 +537,9 @@ struct rq { unsigned long nr_load_updates; u64 nr_switches; + struct load_weight lw_cache[4]; + int lw_cache_idx; + struct cfs_rq cfs; struct rt_rq rt; @@ -1438,8 +1441,24 @@ calc_delta_mine(unsigned long delta_exec { u64 tmp; - if (unlikely(!lw->inv_weight)) - lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1); + if (!lw->inv_weight) { + struct rq *rq = cpu_rq(smp_processor_id()); + unsigned long weight = lw->weight; + int i; + + for (i = 0; i < ARRAY_SIZE(rq->lw_cache); i++) { + if (rq->lw_cache[i].weight == weight) + lw->inv_weight = rq->lw_cache[i].inv_weight; + goto got_inv; + } + if (unlikely(!weight)) + weight++; + lw->inv_weight = 1 + (WMULT_CONST - weight/2) / weight; + rq->lw_cache[rq->lw_cache_idx] = *lw; + rq->lw_cache_idx++; + rq->lw_cache_idx %= ARRAY_SIZE(rq->lw_cache); + } + got_inv: tmp = (u64)delta_exec * weight; /* @@ -8025,7 +8044,7 @@ static void init_tg_cfs_entry(struct tas se->my_q = cfs_rq; se->load.weight = tg->shares; - se->load.inv_weight = div64_64(1ULL<<32, se->load.weight); + se->load.inv_weight = 0; se->parent = parent; } #endif @@ -8692,7 +8711,7 @@ static void __set_se_shares(struct sched dequeue_entity(cfs_rq, se, 0); se->load.weight = shares; - se->load.inv_weight = div64_64((1ULL<<32), shares); + se->load.inv_weight = 0; if (on_rq) enqueue_entity(cfs_rq, se, 0);
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