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sched: use a 2-d bitmap for searching lowest-pri CPU
The current code use a linear algorithm which causes scaling issues on larger SMP machines. This patch replaces that algorithm with a 2-dimensional bitmap to reduce latencies in the wake-up path. Signed-off-by: Gregory Haskins <ghaskins@novell.com> Acked-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Gregory Haskins
Ingo Molnar
parent
f333fdc909
commit
6e0534f278
@ -69,6 +69,7 @@ obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
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obj-$(CONFIG_TASKSTATS) += taskstats.o tsacct.o
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obj-$(CONFIG_MARKERS) += marker.o
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obj-$(CONFIG_LATENCYTOP) += latencytop.o
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obj-$(CONFIG_SMP) += sched_cpupri.o
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ifneq ($(CONFIG_SCHED_NO_NO_OMIT_FRAME_POINTER),y)
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# According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
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@ -74,6 +74,8 @@
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#include <asm/tlb.h>
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#include <asm/irq_regs.h>
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#include "sched_cpupri.h"
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/*
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* Convert user-nice values [ -20 ... 0 ... 19 ]
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* to static priority [ MAX_RT_PRIO..MAX_PRIO-1 ],
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@ -450,6 +452,9 @@ struct root_domain {
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*/
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cpumask_t rto_mask;
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atomic_t rto_count;
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#ifdef CONFIG_SMP
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struct cpupri cpupri;
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#endif
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};
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/*
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@ -6392,6 +6397,8 @@ static void init_rootdomain(struct root_domain *rd)
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cpus_clear(rd->span);
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cpus_clear(rd->online);
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cpupri_init(&rd->cpupri);
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}
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static void init_defrootdomain(void)
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174
kernel/sched_cpupri.c
Normal file
174
kernel/sched_cpupri.c
Normal file
@ -0,0 +1,174 @@
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/*
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* kernel/sched_cpupri.c
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*
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* CPU priority management
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*
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* Copyright (C) 2007-2008 Novell
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*
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* Author: Gregory Haskins <ghaskins@novell.com>
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*
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* This code tracks the priority of each CPU so that global migration
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* decisions are easy to calculate. Each CPU can be in a state as follows:
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*
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* (INVALID), IDLE, NORMAL, RT1, ... RT99
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*
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* going from the lowest priority to the highest. CPUs in the INVALID state
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* are not eligible for routing. The system maintains this state with
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* a 2 dimensional bitmap (the first for priority class, the second for cpus
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* in that class). Therefore a typical application without affinity
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* restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
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* searches). For tasks with affinity restrictions, the algorithm has a
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* worst case complexity of O(min(102, nr_domcpus)), though the scenario that
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* yields the worst case search is fairly contrived.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; version 2
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* of the License.
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*/
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#include "sched_cpupri.h"
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/* Convert between a 140 based task->prio, and our 102 based cpupri */
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static int convert_prio(int prio)
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{
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int cpupri;
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if (prio == CPUPRI_INVALID)
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cpupri = CPUPRI_INVALID;
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else if (prio == MAX_PRIO)
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cpupri = CPUPRI_IDLE;
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else if (prio >= MAX_RT_PRIO)
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cpupri = CPUPRI_NORMAL;
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else
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cpupri = MAX_RT_PRIO - prio + 1;
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return cpupri;
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}
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#define for_each_cpupri_active(array, idx) \
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for (idx = find_first_bit(array, CPUPRI_NR_PRIORITIES); \
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idx < CPUPRI_NR_PRIORITIES; \
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idx = find_next_bit(array, CPUPRI_NR_PRIORITIES, idx+1))
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/**
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* cpupri_find - find the best (lowest-pri) CPU in the system
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* @cp: The cpupri context
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* @p: The task
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* @lowest_mask: A mask to fill in with selected CPUs
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*
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* Note: This function returns the recommended CPUs as calculated during the
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* current invokation. By the time the call returns, the CPUs may have in
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* fact changed priorities any number of times. While not ideal, it is not
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* an issue of correctness since the normal rebalancer logic will correct
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* any discrepancies created by racing against the uncertainty of the current
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* priority configuration.
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*
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* Returns: (int)bool - CPUs were found
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*/
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int cpupri_find(struct cpupri *cp, struct task_struct *p,
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cpumask_t *lowest_mask)
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{
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int idx = 0;
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int task_pri = convert_prio(p->prio);
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for_each_cpupri_active(cp->pri_active, idx) {
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struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
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cpumask_t mask;
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if (idx >= task_pri)
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break;
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cpus_and(mask, p->cpus_allowed, vec->mask);
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if (cpus_empty(mask))
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continue;
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*lowest_mask = mask;
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return 1;
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}
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return 0;
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}
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/**
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* cpupri_set - update the cpu priority setting
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* @cp: The cpupri context
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* @cpu: The target cpu
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* @pri: The priority (INVALID-RT99) to assign to this CPU
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*
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* Note: Assumes cpu_rq(cpu)->lock is locked
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*
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* Returns: (void)
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*/
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void cpupri_set(struct cpupri *cp, int cpu, int newpri)
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{
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int *currpri = &cp->cpu_to_pri[cpu];
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int oldpri = *currpri;
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unsigned long flags;
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newpri = convert_prio(newpri);
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BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
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if (newpri == oldpri)
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return;
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/*
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* If the cpu was currently mapped to a different value, we
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* first need to unmap the old value
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*/
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if (likely(oldpri != CPUPRI_INVALID)) {
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struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri];
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spin_lock_irqsave(&vec->lock, flags);
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vec->count--;
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if (!vec->count)
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clear_bit(oldpri, cp->pri_active);
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cpu_clear(cpu, vec->mask);
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spin_unlock_irqrestore(&vec->lock, flags);
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}
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if (likely(newpri != CPUPRI_INVALID)) {
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struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
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spin_lock_irqsave(&vec->lock, flags);
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cpu_set(cpu, vec->mask);
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vec->count++;
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if (vec->count == 1)
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set_bit(newpri, cp->pri_active);
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spin_unlock_irqrestore(&vec->lock, flags);
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}
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*currpri = newpri;
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}
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/**
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* cpupri_init - initialize the cpupri structure
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* @cp: The cpupri context
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*
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* Returns: (void)
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*/
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void cpupri_init(struct cpupri *cp)
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{
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int i;
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memset(cp, 0, sizeof(*cp));
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for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
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struct cpupri_vec *vec = &cp->pri_to_cpu[i];
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spin_lock_init(&vec->lock);
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vec->count = 0;
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cpus_clear(vec->mask);
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}
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for_each_possible_cpu(i)
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cp->cpu_to_pri[i] = CPUPRI_INVALID;
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}
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36
kernel/sched_cpupri.h
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36
kernel/sched_cpupri.h
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@ -0,0 +1,36 @@
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#ifndef _LINUX_CPUPRI_H
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#define _LINUX_CPUPRI_H
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#include <linux/sched.h>
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#define CPUPRI_NR_PRIORITIES 2+MAX_RT_PRIO
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#define CPUPRI_NR_PRI_WORDS CPUPRI_NR_PRIORITIES/BITS_PER_LONG
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#define CPUPRI_INVALID -1
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#define CPUPRI_IDLE 0
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#define CPUPRI_NORMAL 1
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/* values 2-101 are RT priorities 0-99 */
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struct cpupri_vec {
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spinlock_t lock;
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int count;
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cpumask_t mask;
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};
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struct cpupri {
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struct cpupri_vec pri_to_cpu[CPUPRI_NR_PRIORITIES];
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long pri_active[CPUPRI_NR_PRI_WORDS];
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int cpu_to_pri[NR_CPUS];
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};
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#ifdef CONFIG_SMP
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int cpupri_find(struct cpupri *cp,
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struct task_struct *p, cpumask_t *lowest_mask);
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void cpupri_set(struct cpupri *cp, int cpu, int pri);
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void cpupri_init(struct cpupri *cp);
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#else
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#define cpupri_set(cp, cpu, pri) do { } while (0)
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#define cpupri_init() do { } while (0)
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#endif
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#endif /* _LINUX_CPUPRI_H */
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@ -391,8 +391,11 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
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WARN_ON(!rt_prio(rt_se_prio(rt_se)));
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rt_rq->rt_nr_running++;
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#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
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if (rt_se_prio(rt_se) < rt_rq->highest_prio)
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if (rt_se_prio(rt_se) < rt_rq->highest_prio) {
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struct rq *rq = rq_of_rt_rq(rt_rq);
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rt_rq->highest_prio = rt_se_prio(rt_se);
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cpupri_set(&rq->rd->cpupri, rq->cpu, rt_se_prio(rt_se));
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}
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#endif
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#ifdef CONFIG_SMP
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if (rt_se->nr_cpus_allowed > 1) {
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@ -416,6 +419,10 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
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static inline
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void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
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{
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#ifdef CONFIG_SMP
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int highest_prio = rt_rq->highest_prio;
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#endif
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WARN_ON(!rt_prio(rt_se_prio(rt_se)));
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WARN_ON(!rt_rq->rt_nr_running);
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rt_rq->rt_nr_running--;
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@ -439,6 +446,11 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
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rq->rt.rt_nr_migratory--;
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}
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if (rt_rq->highest_prio != highest_prio) {
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struct rq *rq = rq_of_rt_rq(rt_rq);
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cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio);
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}
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update_rt_migration(rq_of_rt_rq(rt_rq));
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#endif /* CONFIG_SMP */
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#ifdef CONFIG_RT_GROUP_SCHED
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@ -763,73 +775,6 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
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static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
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static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
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{
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int lowest_prio = -1;
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int lowest_cpu = -1;
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int count = 0;
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int cpu;
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cpus_and(*lowest_mask, task_rq(task)->rd->online, task->cpus_allowed);
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/*
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* Scan each rq for the lowest prio.
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*/
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for_each_cpu_mask(cpu, *lowest_mask) {
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struct rq *rq = cpu_rq(cpu);
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/* We look for lowest RT prio or non-rt CPU */
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if (rq->rt.highest_prio >= MAX_RT_PRIO) {
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/*
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* if we already found a low RT queue
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* and now we found this non-rt queue
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* clear the mask and set our bit.
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* Otherwise just return the queue as is
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* and the count==1 will cause the algorithm
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* to use the first bit found.
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*/
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if (lowest_cpu != -1) {
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cpus_clear(*lowest_mask);
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cpu_set(rq->cpu, *lowest_mask);
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}
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return 1;
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}
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/* no locking for now */
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if ((rq->rt.highest_prio > task->prio)
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&& (rq->rt.highest_prio >= lowest_prio)) {
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if (rq->rt.highest_prio > lowest_prio) {
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/* new low - clear old data */
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lowest_prio = rq->rt.highest_prio;
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lowest_cpu = cpu;
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count = 0;
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}
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count++;
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} else
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cpu_clear(cpu, *lowest_mask);
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}
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/*
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* Clear out all the set bits that represent
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* runqueues that were of higher prio than
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* the lowest_prio.
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*/
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if (lowest_cpu > 0) {
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/*
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* Perhaps we could add another cpumask op to
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* zero out bits. Like cpu_zero_bits(cpumask, nrbits);
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* Then that could be optimized to use memset and such.
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*/
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for_each_cpu_mask(cpu, *lowest_mask) {
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if (cpu >= lowest_cpu)
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break;
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cpu_clear(cpu, *lowest_mask);
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}
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}
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return count;
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}
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static inline int pick_optimal_cpu(int this_cpu, cpumask_t *mask)
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{
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int first;
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@ -851,18 +796,13 @@ static int find_lowest_rq(struct task_struct *task)
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cpumask_t *lowest_mask = &__get_cpu_var(local_cpu_mask);
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int this_cpu = smp_processor_id();
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int cpu = task_cpu(task);
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int count = find_lowest_cpus(task, lowest_mask);
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if (!count)
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if (task->rt.nr_cpus_allowed == 1)
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return -1; /* No other targets possible */
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if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
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return -1; /* No targets found */
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/*
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* There is no sense in performing an optimal search if only one
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* target is found.
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*/
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if (count == 1)
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return first_cpu(*lowest_mask);
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/*
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* At this point we have built a mask of cpus representing the
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* lowest priority tasks in the system. Now we want to elect
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@ -1218,6 +1158,8 @@ static void join_domain_rt(struct rq *rq)
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{
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if (rq->rt.overloaded)
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rt_set_overload(rq);
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cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
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}
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/* Assumes rq->lock is held */
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@ -1225,6 +1167,8 @@ static void leave_domain_rt(struct rq *rq)
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{
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if (rq->rt.overloaded)
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rt_clear_overload(rq);
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cpupri_set(&rq->rd->cpupri, rq->cpu, CPUPRI_INVALID);
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}
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/*
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