Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758081Ab2JXJ6I (ORCPT ); Wed, 24 Oct 2012 05:58:08 -0400 Received: from terminus.zytor.com ([198.137.202.10]:55047 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755206Ab2JXJ6F (ORCPT ); Wed, 24 Oct 2012 05:58:05 -0400 Date: Wed, 24 Oct 2012 02:56:39 -0700 From: tip-bot for Paul Turner Message-ID: Cc: linux-kernel@vger.kernel.org, bsegall@google.com, hpa@zytor.com, mingo@kernel.org, a.p.zijlstra@chello.nl, pjt@google.com, tglx@linutronix.de Reply-To: mingo@kernel.org, hpa@zytor.com, bsegall@google.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, pjt@google.com, tglx@linutronix.de In-Reply-To: <20120823141507.277808946@google.com> References: <20120823141507.277808946@google.com> To: linux-tip-commits@vger.kernel.org Subject: [tip:sched/core] sched: Make __update_entity_runnable_avg() fast Git-Commit-ID: 5b51f2f80b3b906ce59bd4dce6eca3c7f34cb1b9 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.6 (terminus.zytor.com [127.0.0.1]); Wed, 24 Oct 2012 02:56:45 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7797 Lines: 264 Commit-ID: 5b51f2f80b3b906ce59bd4dce6eca3c7f34cb1b9 Gitweb: http://git.kernel.org/tip/5b51f2f80b3b906ce59bd4dce6eca3c7f34cb1b9 Author: Paul Turner AuthorDate: Thu, 4 Oct 2012 13:18:32 +0200 Committer: Ingo Molnar CommitDate: Wed, 24 Oct 2012 10:27:30 +0200 sched: Make __update_entity_runnable_avg() fast __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. C program to generate the magic constants in the arrays: #include #include #define N 32 #define WMULT_SHIFT 32 const long WMULT_CONST = ((1UL << N) - 1); double y; long runnable_avg_yN_inv[N]; void calc_mult_inv() { int i; double yn = 0; printf("inverses\n"); for (i = 0; i < N; i++) { yn = (double)WMULT_CONST * pow(y, i); runnable_avg_yN_inv[i] = yn; printf("%2d: 0x%8lx\n", i, runnable_avg_yN_inv[i]); } printf("\n"); } long mult_inv(long c, int n) { return (c * runnable_avg_yN_inv[n]) >> WMULT_SHIFT; } void calc_yn_sum(int n) { int i; double sum = 0, sum_fl = 0, diff = 0; /* * We take the floored sum to ensure the sum of partial sums is never * larger than the actual sum. */ printf("sum y^n\n"); printf(" %8s %8s %8s\n", "exact", "floor", "error"); for (i = 1; i <= n; i++) { sum = (y * sum + y * 1024); sum_fl = floor(y * sum_fl+ y * 1024); printf("%2d: %8.0f %8.0f %8.0f\n", i, sum, sum_fl, sum_fl - sum); } printf("\n"); } void calc_conv(long n) { long old_n; int i = -1; printf("convergence (LOAD_AVG_MAX, LOAD_AVG_MAX_N)\n"); do { old_n = n; n = mult_inv(n, 1) + 1024; i++; } while (n != old_n); printf("%d> %ld\n", i - 1, n); printf("\n"); } void main() { y = pow(0.5, 1/(double)N); calc_mult_inv(); calc_conv(1024); calc_yn_sum(N); } [ Compile with -lm ] Signed-off-by: Paul Turner Reviewed-by: Ben Segall Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/20120823141507.277808946@google.com Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 125 +++++++++++++++++++++++++++++++++++++++++---------- 1 files changed, 101 insertions(+), 24 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 002a769..6ecf455 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -884,17 +884,92 @@ static inline void update_cfs_shares(struct cfs_rq *cfs_rq) #ifdef 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 +#define LOAD_AVG_MAX 47742 /* maximum possible load avg */ +#define LOAD_AVG_MAX_N 345 /* number of full periods to produce LOAD_MAX_AVG */ + +/* Precomputed fixed inverse multiplies for multiplication by y^n */ +static const u32 runnable_avg_yN_inv[] = { + 0xffffffff, 0xfa83b2da, 0xf5257d14, 0xefe4b99a, 0xeac0c6e6, 0xe5b906e6, + 0xe0ccdeeb, 0xdbfbb796, 0xd744fcc9, 0xd2a81d91, 0xce248c14, 0xc9b9bd85, + 0xc5672a10, 0xc12c4cc9, 0xbd08a39e, 0xb8fbaf46, 0xb504f333, 0xb123f581, + 0xad583ee9, 0xa9a15ab4, 0xa5fed6a9, 0xa2704302, 0x9ef5325f, 0x9b8d39b9, + 0x9837f050, 0x94f4efa8, 0x91c3d373, 0x8ea4398a, 0x8b95c1e3, 0x88980e80, + 0x85aac367, 0x82cd8698, +}; + +/* + * Precomputed \Sum y^k { 1<=k<=n }. These are floor(true_value) to prevent + * over-estimates when re-combining. + */ +static const 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, u64 n) { - for (; n && val; n--) { - val *= 4008; - val >>= 12; + unsigned int local_n; + + if (!n) + return val; + else if (unlikely(n > LOAD_AVG_PERIOD * 63)) + return 0; + + /* after bounds checking we can collapse to 32-bit */ + local_n = n; + + /* + * 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 >>= local_n / LOAD_AVG_PERIOD; + local_n %= LOAD_AVG_PERIOD; } - return val; + val *= runnable_avg_yN_inv[local_n]; + /* We don't use SRR here since we always want to round down. */ + return 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_MAX_N)) + return LOAD_AVG_MAX; + + /* Compute \Sum k^n combining precomputed values for k^i, \Sum k^j */ + do { + contrib /= 2; /* y^LOAD_AVG_PERIOD = 1/2 */ + contrib += runnable_avg_yN_sum[LOAD_AVG_PERIOD]; + + n -= LOAD_AVG_PERIOD; + } while (n > LOAD_AVG_PERIOD); + + contrib = decay_load(contrib, n); + return contrib + runnable_avg_yN_sum[n]; } /* @@ -929,7 +1004,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now, struct sched_avg *sa, int runnable) { - u64 delta; + u64 delta, periods; + u32 runnable_contrib; int delta_w, decayed = 0; delta = now - sa->last_runnable_update; @@ -963,25 +1039,26 @@ static __always_inline int __update_entity_runnable_avg(u64 now, * period and accrue it. */ 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; - - /* - * Remainder of delta initiates a new period, roll over - * the previous. - */ - 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; - /* New period is empty */ - delta_w = 1024; - } while (delta >= 1024); + if (runnable) + sa->runnable_avg_sum += delta_w; + sa->runnable_avg_period += delta_w; + + delta -= delta_w; + + /* Figure out how many additional periods this update spans */ + 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); + + /* Efficiently calculate \sum (1..n_period) 1024*y^i */ + runnable_contrib = __compute_runnable_contrib(periods); + if (runnable) + sa->runnable_avg_sum += runnable_contrib; + sa->runnable_avg_period += runnable_contrib; } /* Remainder of delta accrued against u_0` */ -- 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/