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sched/fair: Fix calc_cfs_shares() fixed point arithmetics width confusion
Commit:fde7d22e01
("sched/fair: Fix overly small weight for interactive group entities") did something non-obvious but also did it buggy yet latent. The problem was exposed for real by a later commit in the v4.7 merge window:2159197d66
("sched/core: Enable increased load resolution on 64-bit kernels") ... after which tg->load_avg and cfs_rq->load.weight had different units (10 bit fixed point and 20 bit fixed point resp.). Add a comment to explain the use of cfs_rq->load.weight over the 'natural' cfs_rq->avg.load_avg and add scale_load_down() to correct for the difference in unit. Since this is (now, as per a previous commit) the only user of calc_tg_weight(), collapse it. The effects of this bug should be randomly inconsistent SMP-balancing of cgroups workloads. Reported-by: Jirka Hladky <jhladky@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Fixes:2159197d66
("sched/core: Enable increased load resolution on 64-bit kernels") Fixes:fde7d22e01
("sched/fair: Fix overly small weight for interactive group entities") Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -2497,28 +2497,22 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
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#ifdef CONFIG_FAIR_GROUP_SCHED
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# ifdef CONFIG_SMP
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static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq)
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{
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long tg_weight;
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/*
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* Use this CPU's real-time load instead of the last load contribution
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* as the updating of the contribution is delayed, and we will use the
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* the real-time load to calc the share. See update_tg_load_avg().
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*/
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tg_weight = atomic_long_read(&tg->load_avg);
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tg_weight -= cfs_rq->tg_load_avg_contrib;
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tg_weight += cfs_rq->load.weight;
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return tg_weight;
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}
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static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
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{
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long tg_weight, load, shares;
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tg_weight = calc_tg_weight(tg, cfs_rq);
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load = cfs_rq->load.weight;
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/*
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* This really should be: cfs_rq->avg.load_avg, but instead we use
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* cfs_rq->load.weight, which is its upper bound. This helps ramp up
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* the shares for small weight interactive tasks.
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*/
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load = scale_load_down(cfs_rq->load.weight);
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tg_weight = atomic_long_read(&tg->load_avg);
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/* Ensure tg_weight >= load */
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tg_weight -= cfs_rq->tg_load_avg_contrib;
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tg_weight += load;
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shares = (tg->shares * load);
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if (tg_weight)
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@ -2537,6 +2531,7 @@ static inline long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
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return tg->shares;
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}
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# endif /* CONFIG_SMP */
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static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
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unsigned long weight)
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{
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