sched: Fix /proc/sched_stat failure on very very large systems

On systems with 4096 cores doing a cat /proc/sched_stat fails,
because we are trying to push all the data into a single kmalloc
buffer.

The issue is on these very large machines all the data will not
fit in 4mb.

A better solution is to not use the single_open() mechanism but
to provide our own seq_operations.

The output should be identical to previous version and thus not
need the version number.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
[ Fix memleak]
[ Fix spello in comment]
[ Fix warnings]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/stats.c b/kernel/sched/stats.c
index 903ffa9..e036eda 100644 (file)
--- a/kernel/sched/stats.c
+++ b/kernel/sched/stats.c
@@ -21,14 +21,17 @@ static int show_schedstat(struct seq_file *seq, void *v)
        if (mask_str == NULL)
                return -ENOMEM;
 
-       seq_printf(seq, "version %d\n", SCHEDSTAT_VERSION);
-       seq_printf(seq, "timestamp %lu\n", jiffies);
-       for_each_online_cpu(cpu) {
-               struct rq *rq = cpu_rq(cpu);
+       if (v == (void *)1) {
+               seq_printf(seq, "version %d\n", SCHEDSTAT_VERSION);
+               seq_printf(seq, "timestamp %lu\n", jiffies);
+       } else {
+               struct rq *rq;
 #ifdef CONFIG_SMP
                struct sched_domain *sd;
                int dcount = 0;
 #endif
+               cpu = (unsigned long)(v - 2);
+               rq = cpu_rq(cpu);
 
                /* runqueue-specific stats */
                seq_printf(seq,
@@ -77,30 +80,66 @@ static int show_schedstat(struct seq_file *seq, void *v)
        return 0;
 }
 
-static int schedstat_open(struct inode *inode, struct file *file)
+/*
+ * This itererator needs some explanation.
+ * It returns 1 for the header position.
+ * This means 2 is cpu 0.
+ * In a hotplugged system some cpus, including cpu 0, may be missing so we have
+ * to use cpumask_* to iterate over the cpus.
+ */
+static void *schedstat_start(struct seq_file *file, loff_t *offset)
 {
-       unsigned int size = PAGE_SIZE * (1 + num_online_cpus() / 32);
-       char *buf = kmalloc(size, GFP_KERNEL);
-       struct seq_file *m;
-       int res;
+       unsigned long n = *offset;
 
-       if (!buf)
-               return -ENOMEM;
-       res = single_open(file, show_schedstat, NULL);
-       if (!res) {
-               m = file->private_data;
-               m->buf = buf;
-               m->size = size;
-       } else
-               kfree(buf);
-       return res;
+       if (n == 0)
+               return (void *) 1;
+
+       n--;
+
+       if (n > 0)
+               n = cpumask_next(n - 1, cpu_online_mask);
+       else
+               n = cpumask_first(cpu_online_mask);
+
+       *offset = n + 1;
+
+       if (n < nr_cpu_ids)
+               return (void *)(unsigned long)(n + 2);
+       return NULL;
+}
+
+static void *schedstat_next(struct seq_file *file, void *data, loff_t *offset)
+{
+       (*offset)++;
+       return schedstat_start(file, offset);
+}
+
+static void schedstat_stop(struct seq_file *file, void *data)
+{
+}
+
+static const struct seq_operations schedstat_sops = {
+       .start = schedstat_start,
+       .next  = schedstat_next,
+       .stop  = schedstat_stop,
+       .show  = show_schedstat,
+};
+
+static int schedstat_open(struct inode *inode, struct file *file)
+{
+       return seq_open(file, &schedstat_sops);
 }
 
+static int schedstat_release(struct inode *inode, struct file *file)
+{
+       return 0;
+};
+
 static const struct file_operations proc_schedstat_operations = {
        .open    = schedstat_open,
        .read    = seq_read,
        .llseek  = seq_lseek,
-       .release = single_release,
+       .release = schedstat_release,
 };
 
 static int __init proc_schedstat_init(void)