Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754277Ab2BBBnw (ORCPT ); Wed, 1 Feb 2012 20:43:52 -0500 Received: from mail-ee0-f74.google.com ([74.125.83.74]:41910 "EHLO mail-ee0-f74.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754139Ab2BBBnX (ORCPT ); Wed, 1 Feb 2012 20:43:23 -0500 Subject: [RFC PATCH 13/14] sched: make __update_entity_runnable_avg() fast To: linux-kernel@vger.kernel.org From: Paul Turner Cc: Venki Pallipadi , Srivatsa Vaddagiri , Peter Zijlstra , Mike Galbraith , Kamalesh Babulal , Ben Segall , Ingo Molnar , Vaidyanathan Srinivasan Date: Wed, 01 Feb 2012 17:38:27 -0800 Message-ID: <20120202013827.20844.30497.stgit@kitami.mtv.corp.google.com> In-Reply-To: <20120202013825.20844.26081.stgit@kitami.mtv.corp.google.com> References: <20120202013825.20844.26081.stgit@kitami.mtv.corp.google.com> User-Agent: StGit/0.15 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4999 Lines: 158 __update_entity_runnable_avg forms the core of maintaining an entity's runnable load average. In this function we charge the accumulated run-time since last update and handle appropriate decay. In some cases, e.g. a waking task, this time interval may be much larger than our period unit. Fortunately we can exploit some properties of our series to perform decay for a blocked update in constant time and account the contribution for a running update in essentially-constant* time. [*]: For any running entity they should be performing updates at the tick which gives us a soft limit of 1 jiffy between updates, and we can compute up to a 32 jiffy update in a single pass. Signed-off-by: Paul Turner Signed-off-by: Ben Segall --- kernel/sched/fair.c | 100 ++++++++++++++++++++++++++++++++++++++++++--------- 1 files changed, 82 insertions(+), 18 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 9e2c9a4..ad524bb 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -896,17 +896,77 @@ static inline void update_cfs_shares(struct cfs_rq *cfs_rq) #if defined(CONFIG_FAIR_GROUP_SCHED) && defined(CONFIG_SMP) /* + * We choose a half-life close to 1 scheduling period. + * Note: The tables below are dependent on this value. + */ +#define LOAD_AVG_PERIOD 32 + +/* Precomputed fixed inverse multiplies for multiplication by y^n */ +const static u32 runnable_avg_yN_inv[] = { + 0xffffffff,0xfa83b2db,0xf5257d15,0xefe4b99b,0xeac0c6e7,0xe5b906e7, + 0xe0ccdeec,0xdbfbb797,0xd744fcca,0xd2a81d91,0xce248c15,0xc9b9bd86, + 0xc5672a11,0xc12c4cca,0xbd08a39f,0xb8fbaf47,0xb504f333,0xb123f581, + 0xad583eea,0xa9a15ab4,0xa5fed6a9,0xa2704303,0x9ef53260,0x9b8d39b9, + 0x9837f051,0x94f4efa8,0x91c3d373,0x8ea4398b,0x8b95c1e3,0x88980e80, + 0x85aac367,0x82cd8698, +}; + +/* Precomputed \Sum y^k { 1<=k<=n } */ +const static u32 runnable_avg_yN_sum[] = { + 0, 1002, 1982, 2941, 3880, 4798, 5697, 6576, 7437, 8279, 9103, + 9909,10698,11470,12226,12966,13690,14398,15091,15769,16433,17082, + 17718,18340,18949,19545,20128,20698,21256,21802,22336,22859,23371, +}; + +/* * Approximate: * val * y^n, where y^32 ~= 0.5 (~1 scheduling period) */ static __always_inline u64 decay_load(u64 val, int n) { - for (;n && val;n--) { - val *= 4008; - val >>= 12; + if (!n) + return val; + + /* + * As y^PERIOD = 1/2, we can combine + * y^n = 1/2^(n/PERIOD) * k^(n%PERIOD) + * With a look-up table which covers k^n (n= LOAD_AVG_PERIOD)) { + val >>= n/LOAD_AVG_PERIOD; + n %= LOAD_AVG_PERIOD; } - return val; + val *= runnable_avg_yN_inv[n]; + return SRR(val, 32); +} + +/* + * For updates fully spanning n periods, the contribution to runnable + * average will be: \Sum 1024*y^n + * + * We can compute this reasonably efficiently by combining: + * y^PERIOD = 1/2 with precomputed \Sum 1024*y^n {for n LOAD_AVG_PERIOD); + + contrib = decay_load(contrib, n); + return contrib + runnable_avg_yN_sum[n]; } /* We can represent the historical contribution to runnable average as the @@ -940,6 +1000,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int runnable) { u64 delta; + u32 periods, runnable_contrib; int delta_w, decayed = 0; delta = now - sa->last_runnable_update; @@ -963,20 +1024,23 @@ static __always_inline int __update_entity_runnable_avg(u64 now, decayed = 1; delta_w = 1024 - delta_w; - BUG_ON(delta_w > delta); - do { - if (runnable) - sa->runnable_avg_sum += delta_w; - sa->runnable_avg_period += delta_w; - - sa->runnable_avg_sum = - decay_load(sa->runnable_avg_sum, 1); - sa->runnable_avg_period = - decay_load(sa->runnable_avg_period, 1); - - delta -= delta_w; - delta_w = 1024; - } while (delta >= 1024); + if (runnable) + sa->runnable_avg_sum += delta_w; + sa->runnable_avg_period += delta_w; + + delta -= delta_w; + periods = delta / 1024; + delta %= 1024; + + sa->runnable_avg_sum = decay_load(sa->runnable_avg_sum, + periods + 1); + sa->runnable_avg_period = decay_load(sa->runnable_avg_period, + periods + 1); + + runnable_contrib = __compute_runnable_contrib(periods); + if (runnable) + sa->runnable_avg_sum += runnable_contrib; + sa->runnable_avg_period += runnable_contrib; } if (runnable) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/