Re: [patch 14/16] sched: make __update_entity_runnable_avg() fast
From: Namhyung Kim
Date: Fri Aug 24 2012 - 04:35:10 EST
On Thu, 23 Aug 2012 07:14:36 -0700, > From: Paul Turner <pjt@xxxxxxxxxx>
>
> __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 <pjt@xxxxxxxxxx>
> Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
> ---
> kernel/sched/fair.c | 123 +++++++++++++++++++++++++++++++++++++++++----------
> 1 files changed, 99 insertions(+), 24 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 12e9ae5..b249371 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -879,17 +879,90 @@ 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 47765 /* 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;
> + int local_n;
> + if (!n)
> + return val;
> + else if (unlikely(n > LOAD_AVG_PERIOD * 63))
> + return 0;
> +
> + /* will be 32 bits if that's desirable */
> + 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<PERIOD)
> + *
> + * To achieve constant time decay_load.
> + */
> + if (unlikely(local_n >= LOAD_AVG_PERIOD)) {
> + val >>= local_n / LOAD_AVG_PERIOD;
> + n %= LOAD_AVG_PERIOD;
s/n/local_n/ ?
Thanks,
Namhyung
> }
>
> - return val;
> + val *= runnable_avg_yN_inv[local_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 <PERIOD}
> + */
> +static u32 __compute_runnable_contrib(u64 n)
> +{
> + u32 contrib = 0;
> +
> + if (likely(n <= LOAD_AVG_PERIOD))
> + return runnable_avg_yN_sum[n];
> + else if (unlikely(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];
> }
>
> /* We can represent the historical contribution to runnable average as the
> @@ -923,7 +996,8 @@ 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;
> @@ -957,25 +1031,26 @@ 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` */
--
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