cpuset sched_load_balance flag

Add a new per-cpuset flag called 'sched_load_balance'.

When enabled in a cpuset (the default value) it tells the kernel scheduler
that the scheduler should provide the normal load balancing on the CPUs in
that cpuset, sometimes moving tasks from one CPU to a second CPU if the
second CPU is less loaded and if that task is allowed to run there.

When disabled (write "0" to the file) then it tells the kernel scheduler
that load balancing is not required for the CPUs in that cpuset.

Now even if this flag is disabled for some cpuset, the kernel may still
have to load balance some or all the CPUs in that cpuset, if some
overlapping cpuset has its sched_load_balance flag enabled.

If there are some CPUs that are not in any cpuset whose sched_load_balance
flag is enabled, the kernel scheduler will not load balance tasks to those
CPUs.

Moreover the kernel will partition the 'sched domains' (non-overlapping
sets of CPUs over which load balancing is attempted) into the finest
granularity partition that it can find, while still keeping any two CPUs
that are in the same shed_load_balance enabled cpuset in the same element
of the partition.

This serves two purposes:
 1) It provides a mechanism for real time isolation of some CPUs, and
 2) it can be used to improve performance on systems with many CPUs
    by supporting configurations in which load balancing is not done
    across all CPUs at once, but rather only done in several smaller
    disjoint sets of CPUs.

This mechanism replaces the earlier overloading of the per-cpuset
flag 'cpu_exclusive', which overloading was removed in an earlier
patch: cpuset-remove-sched-domain-hooks-from-cpusets

See further the Documentation and comments in the code itself.

[akpm@linux-foundation.org: don't be weird]
Signed-off-by: Paul Jackson <pj@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 270 insertions, 5 deletions

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 1133062395e2..203ca52e78dd 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -4,7 +4,7 @@
  *  Processor and Memory placement constraints for sets of tasks.
  *
  *  Copyright (C) 2003 BULL SA.
- *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
+ *  Copyright (C) 2004-2007 Silicon Graphics, Inc.
  *  Copyright (C) 2006 Google, Inc
  *
  *  Portions derived from Patrick Mochel's sysfs code.
@@ -54,6 +54,7 @@
 #include <asm/uaccess.h>
 #include <asm/atomic.h>
 #include <linux/mutex.h>
+#include <linux/kfifo.h>
 
 /*
  * Tracks how many cpusets are currently defined in system.
@@ -91,6 +92,9 @@ struct cpuset {
         int mems_generation;
 
         struct fmeter fmeter;           /* memory_pressure filter */
+
+        /* partition number for rebuild_sched_domains() */
+        int pn;
 };
 
 /* Retrieve the cpuset for a cgroup */
@@ -113,6 +117,7 @@ typedef enum {
         CS_CPU_EXCLUSIVE,
         CS_MEM_EXCLUSIVE,
         CS_MEMORY_MIGRATE,
+        CS_SCHED_LOAD_BALANCE,
         CS_SPREAD_PAGE,
         CS_SPREAD_SLAB,
 } cpuset_flagbits_t;
@@ -128,6 +133,11 @@ static inline int is_mem_exclusive(const struct cpuset *cs)
         return test_bit(CS_MEM_EXCLUSIVE, &cs->flags);
 }
 
+static inline int is_sched_load_balance(const struct cpuset *cs)
+{
+        return test_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
+}
+
 static inline int is_memory_migrate(const struct cpuset *cs)
 {
         return test_bit(CS_MEMORY_MIGRATE, &cs->flags);
@@ -482,6 +492,208 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
 }
 
 /*
+ * Helper routine for rebuild_sched_domains().
+ * Do cpusets a, b have overlapping cpus_allowed masks?
+ */
+
+static int cpusets_overlap(struct cpuset *a, struct cpuset *b)
+{
+        return cpus_intersects(a->cpus_allowed, b->cpus_allowed);
+}
+
+/*
+ * rebuild_sched_domains()
+ *
+ * If the flag 'sched_load_balance' of any cpuset with non-empty
+ * 'cpus' changes, or if the 'cpus' allowed changes in any cpuset
+ * which has that flag enabled, or if any cpuset with a non-empty
+ * 'cpus' is removed, then call this routine to rebuild the
+ * scheduler's dynamic sched domains.
+ *
+ * This routine builds a partial partition of the systems CPUs
+ * (the set of non-overlappping cpumask_t's in the array 'part'
+ * below), and passes that partial partition to the kernel/sched.c
+ * partition_sched_domains() routine, which will rebuild the
+ * schedulers load balancing domains (sched domains) as specified
+ * by that partial partition.  A 'partial partition' is a set of
+ * non-overlapping subsets whose union is a subset of that set.
+ *
+ * See "What is sched_load_balance" in Documentation/cpusets.txt
+ * for a background explanation of this.
+ *
+ * Does not return errors, on the theory that the callers of this
+ * routine would rather not worry about failures to rebuild sched
+ * domains when operating in the severe memory shortage situations
+ * that could cause allocation failures below.
+ *
+ * Call with cgroup_mutex held.  May take callback_mutex during
+ * call due to the kfifo_alloc() and kmalloc() calls.  May nest
+ * a call to the lock_cpu_hotplug()/unlock_cpu_hotplug() pair.
+ * Must not be called holding callback_mutex, because we must not
+ * call lock_cpu_hotplug() while holding callback_mutex.  Elsewhere
+ * the kernel nests callback_mutex inside lock_cpu_hotplug() calls.
+ * So the reverse nesting would risk an ABBA deadlock.
+ *
+ * The three key local variables below are:
+ *    q  - a kfifo queue of cpuset pointers, used to implement a
+ *         top-down scan of all cpusets.  This scan loads a pointer
+ *         to each cpuset marked is_sched_load_balance into the
+ *         array 'csa'.  For our purposes, rebuilding the schedulers
+ *         sched domains, we can ignore !is_sched_load_balance cpusets.
+ *  csa  - (for CpuSet Array) Array of pointers to all the cpusets
+ *         that need to be load balanced, for convenient iterative
+ *         access by the subsequent code that finds the best partition,
+ *         i.e the set of domains (subsets) of CPUs such that the
+ *         cpus_allowed of every cpuset marked is_sched_load_balance
+ *         is a subset of one of these domains, while there are as
+ *         many such domains as possible, each as small as possible.
+ * doms  - Conversion of 'csa' to an array of cpumasks, for passing to
+ *         the kernel/sched.c routine partition_sched_domains() in a
+ *         convenient format, that can be easily compared to the prior
+ *         value to determine what partition elements (sched domains)
+ *         were changed (added or removed.)
+ *
+ * Finding the best partition (set of domains):
+ *      The triple nested loops below over i, j, k scan over the
+ *      load balanced cpusets (using the array of cpuset pointers in
+ *      csa[]) looking for pairs of cpusets that have overlapping
+ *      cpus_allowed, but which don't have the same 'pn' partition
+ *      number and gives them in the same partition number.  It keeps
+ *      looping on the 'restart' label until it can no longer find
+ *      any such pairs.
+ *
+ *      The union of the cpus_allowed masks from the set of
+ *      all cpusets having the same 'pn' value then form the one
+ *      element of the partition (one sched domain) to be passed to
+ *      partition_sched_domains().
+ */
+
+static void rebuild_sched_domains(void)
+{
+        struct kfifo *q;        /* queue of cpusets to be scanned */
+        struct cpuset *cp;      /* scans q */
+        struct cpuset **csa;    /* array of all cpuset ptrs */
+        int csn;                /* how many cpuset ptrs in csa so far */
+        int i, j, k;            /* indices for partition finding loops */
+        cpumask_t *doms;        /* resulting partition; i.e. sched domains */
+        int ndoms;              /* number of sched domains in result */
+        int nslot;              /* next empty doms[] cpumask_t slot */
+
+        q = NULL;
+        csa = NULL;
+        doms = NULL;
+
+        /* Special case for the 99% of systems with one, full, sched domain */
+        if (is_sched_load_balance(&top_cpuset)) {
+                ndoms = 1;
+                doms = kmalloc(sizeof(cpumask_t), GFP_KERNEL);
+                if (!doms)
+                        goto rebuild;
+                *doms = top_cpuset.cpus_allowed;
+                goto rebuild;
+        }
+
+        q = kfifo_alloc(number_of_cpusets * sizeof(cp), GFP_KERNEL, NULL);
+        if (IS_ERR(q))
+                goto done;
+        csa = kmalloc(number_of_cpusets * sizeof(cp), GFP_KERNEL);
+        if (!csa)
+                goto done;
+        csn = 0;
+
+        cp = &top_cpuset;
+        __kfifo_put(q, (void *)&cp, sizeof(cp));
+        while (__kfifo_get(q, (void *)&cp, sizeof(cp))) {
+                struct cgroup *cont;
+                struct cpuset *child;   /* scans child cpusets of cp */
+                if (is_sched_load_balance(cp))
+                        csa[csn++] = cp;
+                list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
+                        child = cgroup_cs(cont);
+                        __kfifo_put(q, (void *)&child, sizeof(cp));
+                }
+        }
+
+        for (i = 0; i < csn; i++)
+                csa[i]->pn = i;
+        ndoms = csn;
+
+restart:
+        /* Find the best partition (set of sched domains) */
+        for (i = 0; i < csn; i++) {
+                struct cpuset *a = csa[i];
+                int apn = a->pn;
+
+                for (j = 0; j < csn; j++) {
+                        struct cpuset *b = csa[j];
+                        int bpn = b->pn;
+
+                        if (apn != bpn && cpusets_overlap(a, b)) {
+                                for (k = 0; k < csn; k++) {
+                                        struct cpuset *c = csa[k];
+
+                                        if (c->pn == bpn)
+                                                c->pn = apn;
+                                }
+                                ndoms--;        /* one less element */
+                                goto restart;
+                        }
+                }
+        }
+
+        /* Convert <csn, csa> to <ndoms, doms> */
+        doms = kmalloc(ndoms * sizeof(cpumask_t), GFP_KERNEL);
+        if (!doms)
+                goto rebuild;
+
+        for (nslot = 0, i = 0; i < csn; i++) {
+                struct cpuset *a = csa[i];
+                int apn = a->pn;
+
+                if (apn >= 0) {
+                        cpumask_t *dp = doms + nslot;
+
+                        if (nslot == ndoms) {
+                                static int warnings = 10;
+                                if (warnings) {
+                                        printk(KERN_WARNING
+                                         "rebuild_sched_domains confused:"
+                                          " nslot %d, ndoms %d, csn %d, i %d,"
+                                          " apn %d\n",
+                                          nslot, ndoms, csn, i, apn);
+                                        warnings--;
+                                }
+                                continue;
+                        }
+
+                        cpus_clear(*dp);
+                        for (j = i; j < csn; j++) {
+                                struct cpuset *b = csa[j];
+
+                                if (apn == b->pn) {
+                                        cpus_or(*dp, *dp, b->cpus_allowed);
+                                        b->pn = -1;
+                                }
+                        }
+                        nslot++;
+                }
+        }
+        BUG_ON(nslot != ndoms);
+
+rebuild:
+        /* Have scheduler rebuild sched domains */
+        lock_cpu_hotplug();
+        partition_sched_domains(ndoms, doms);
+        unlock_cpu_hotplug();
+
+done:
+        if (q && !IS_ERR(q))
+                kfifo_free(q);
+        kfree(csa);
+        /* Don't kfree(doms) -- partition_sched_domains() does that. */
+}
+
+/*
  * Call with manage_mutex held.  May take callback_mutex during call.
  */
 
@@ -489,6 +701,7 @@ static int update_cpumask(struct cpuset *cs, char *buf)
 {
         struct cpuset trialcs;
         int retval;
+        int cpus_changed, is_load_balanced;
 
         /* top_cpuset.cpus_allowed tracks cpu_online_map; it's read-only */
         if (cs == &top_cpuset)
@@ -516,9 +729,17 @@ static int update_cpumask(struct cpuset *cs, char *buf)
         retval = validate_change(cs, &trialcs);
         if (retval < 0)
                 return retval;
+
+        cpus_changed = !cpus_equal(cs->cpus_allowed, trialcs.cpus_allowed);
+        is_load_balanced = is_sched_load_balance(&trialcs);
+
         mutex_lock(&callback_mutex);
         cs->cpus_allowed = trialcs.cpus_allowed;
         mutex_unlock(&callback_mutex);
+
+        if (cpus_changed && is_load_balanced)
+                rebuild_sched_domains();
+
         return 0;
 }
 
@@ -752,6 +973,7 @@ static int update_memory_pressure_enabled(struct cpuset *cs, char *buf)
 /*
  * update_flag - read a 0 or a 1 in a file and update associated flag
  * bit: the bit to update (CS_CPU_EXCLUSIVE, CS_MEM_EXCLUSIVE,
+ *                              CS_SCHED_LOAD_BALANCE,
  *                              CS_NOTIFY_ON_RELEASE, CS_MEMORY_MIGRATE,
  *                              CS_SPREAD_PAGE, CS_SPREAD_SLAB)
  * cs:  the cpuset to update
@@ -765,6 +987,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs, char *buf)
         int turning_on;
         struct cpuset trialcs;
         int err;
+        int cpus_nonempty, balance_flag_changed;
 
         turning_on = (simple_strtoul(buf, NULL, 10) != 0);
 
@@ -777,10 +1000,18 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs, char *buf)
         err = validate_change(cs, &trialcs);
         if (err < 0)
                 return err;
+
+        cpus_nonempty = !cpus_empty(trialcs.cpus_allowed);
+        balance_flag_changed = (is_sched_load_balance(cs) !=
+                                        is_sched_load_balance(&trialcs));
+
         mutex_lock(&callback_mutex);
         cs->flags = trialcs.flags;
         mutex_unlock(&callback_mutex);
 
+        if (cpus_nonempty && balance_flag_changed)
+                rebuild_sched_domains();
+
         return 0;
 }
 
@@ -928,6 +1159,7 @@ typedef enum {
         FILE_MEMLIST,
         FILE_CPU_EXCLUSIVE,
         FILE_MEM_EXCLUSIVE,
+        FILE_SCHED_LOAD_BALANCE,
         FILE_MEMORY_PRESSURE_ENABLED,
         FILE_MEMORY_PRESSURE,
         FILE_SPREAD_PAGE,
@@ -946,7 +1178,7 @@ static ssize_t cpuset_common_file_write(struct cgroup *cont,
         int retval = 0;
 
         /* Crude upper limit on largest legitimate cpulist user might write. */
-        if (nbytes > 100 + 6 * max(NR_CPUS, MAX_NUMNODES))
+        if (nbytes > 100U + 6 * max(NR_CPUS, MAX_NUMNODES))
                 return -E2BIG;
 
         /* +1 for nul-terminator */
@@ -979,6 +1211,9 @@ static ssize_t cpuset_common_file_write(struct cgroup *cont,
         case FILE_MEM_EXCLUSIVE:
                 retval = update_flag(CS_MEM_EXCLUSIVE, cs, buffer);
                 break;
+        case FILE_SCHED_LOAD_BALANCE:
+                retval = update_flag(CS_SCHED_LOAD_BALANCE, cs, buffer);
+                break;
         case FILE_MEMORY_MIGRATE:
                 retval = update_flag(CS_MEMORY_MIGRATE, cs, buffer);
                 break;
@@ -1074,6 +1309,9 @@ static ssize_t cpuset_common_file_read(struct cgroup *cont,
         case FILE_MEM_EXCLUSIVE:
                 *s++ = is_mem_exclusive(cs) ? '1' : '0';
                 break;
+        case FILE_SCHED_LOAD_BALANCE:
+                *s++ = is_sched_load_balance(cs) ? '1' : '0';
+                break;
         case FILE_MEMORY_MIGRATE:
                 *s++ = is_memory_migrate(cs) ? '1' : '0';
                 break;
@@ -1137,6 +1375,13 @@ static struct cftype cft_mem_exclusive = {
         .private = FILE_MEM_EXCLUSIVE,
 };
 
+static struct cftype cft_sched_load_balance = {
+        .name = "sched_load_balance",
+        .read = cpuset_common_file_read,
+        .write = cpuset_common_file_write,
+        .private = FILE_SCHED_LOAD_BALANCE,
+};
+
 static struct cftype cft_memory_migrate = {
         .name = "memory_migrate",
         .read = cpuset_common_file_read,
@@ -1186,6 +1431,8 @@ static int cpuset_populate(struct cgroup_subsys *ss, struct cgroup *cont)
                 return err;
         if ((err = cgroup_add_file(cont, ss, &cft_memory_migrate)) < 0)
                 return err;
+        if ((err = cgroup_add_file(cont, ss, &cft_sched_load_balance)) < 0)
+                return err;
         if ((err = cgroup_add_file(cont, ss, &cft_memory_pressure)) < 0)
                 return err;
         if ((err = cgroup_add_file(cont, ss, &cft_spread_page)) < 0)
@@ -1267,6 +1514,7 @@ static struct cgroup_subsys_state *cpuset_create(
                 set_bit(CS_SPREAD_PAGE, &cs->flags);
         if (is_spread_slab(parent))
                 set_bit(CS_SPREAD_SLAB, &cs->flags);
+        set_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
         cs->cpus_allowed = CPU_MASK_NONE;
         cs->mems_allowed = NODE_MASK_NONE;
         cs->mems_generation = cpuset_mems_generation++;
@@ -1277,11 +1525,27 @@ static struct cgroup_subsys_state *cpuset_create(
         return &cs->css ;
 }
 
+/*
+ * Locking note on the strange update_flag() call below:
+ *
+ * If the cpuset being removed has its flag 'sched_load_balance'
+ * enabled, then simulate turning sched_load_balance off, which
+ * will call rebuild_sched_domains().  The lock_cpu_hotplug()
+ * call in rebuild_sched_domains() must not be made while holding
+ * callback_mutex.  Elsewhere the kernel nests callback_mutex inside
+ * lock_cpu_hotplug() calls.  So the reverse nesting would risk an
+ * ABBA deadlock.
+ */
+
 static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
 {
         struct cpuset *cs = cgroup_cs(cont);
 
         cpuset_update_task_memory_state();
+
+        if (is_sched_load_balance(cs))
+                update_flag(CS_SCHED_LOAD_BALANCE, cs, "0");
+
         number_of_cpusets--;
         kfree(cs);
 }
@@ -1326,6 +1590,7 @@ int __init cpuset_init(void)
 
         fmeter_init(&top_cpuset.fmeter);
         top_cpuset.mems_generation = cpuset_mems_generation++;
+        set_bit(CS_SCHED_LOAD_BALANCE, &top_cpuset.flags);
 
         err = register_filesystem(&cpuset_fs_type);
         if (err < 0)
@@ -1412,8 +1677,8 @@ static void common_cpu_mem_hotplug_unplug(void)
  * cpu_online_map on each CPU hotplug (cpuhp) event.
  */
 
-static int cpuset_handle_cpuhp(struct notifier_block *nb,
-                                unsigned long phase, void *cpu)
+static int cpuset_handle_cpuhp(struct notifier_block *unused_nb,
+                                unsigned long phase, void *unused_cpu)
 {
         if (phase == CPU_DYING || phase == CPU_DYING_FROZEN)
                 return NOTIFY_DONE;
@@ -1803,7 +2068,7 @@ void __cpuset_memory_pressure_bump(void)
  *    the_top_cpuset_hack in cpuset_exit(), which sets an exiting tasks
  *    cpuset to top_cpuset.
  */
-static int proc_cpuset_show(struct seq_file *m, void *v)
+static int proc_cpuset_show(struct seq_file *m, void *unused_v)
 {
         struct pid *pid;
         struct task_struct *tsk;