From 4161cb2d867b2210b447afc4a941ab4b96e1bd13 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 12 Mar 2026 14:48:44 +0100 Subject: [PATCH] sched/fair: Add cgroup_mode switch The effective task weight (W_t') for a task in cgroup g on CPU n is given by: W_t W_t' = W_g * F_g_n * ---------- \Sum W_t_n Where W_g is the group's weight (cpu.weight), F_g_n is the fraction of the group weight for CPU n and W_t/W is the relative weight of this task against all other tasks in the same group on the same CPU. Furthermore, this makes: \Sum W_t_n F_g_n = ---------- \Sum W_t The fraction of weight inside the group of CPU n against the whole group. The problem is with F_g_n, the primary goal of this fraction is to make sure that the relative weight of tasks, when distributed over CPUs is maintained. For example, consider 4 (equal weight) tasks and 2 CPUs with a 1:3 distribution, then if F_g_n would simply be 1 (no weight re-distribution) the effective relative weights (W_t') of the tasks in our group would be: CPU0 CPU1 W_g W_g/3 W_g/3 W_g/3 IOW, the lucky task on CPU0 would get an equal amount of weight as all 3 tasks on CPU1 combined. However, with the weight redistribution, this becomes: CPU0 CPU1 W_g/4 W_g/4 W_g/4 W_g/4 All tasks are equal weight (as intended). However, as is already evident from this example, the more CPUs you add, the smaller F_g_n becomes, which creates a disparity against tasks not in our group. Specifically: avg(F_g_n) ~ 1/N This leads to a weight mismatch in the hierarchy. IOW tasks cannot compete fairly across hierarchy levels. *Notably*, what is meant by avg(F_g_n) being proportional to 1/N is that when there are at least N runnable tasks, the average of this fraction tends to 1/N. For a hierarchy of depth d, this gets even worse, since that gets terms on the order of: avg(F_g_n)^d ~ 1/(N^d) Given fixed point arithmetic, this also leads to numerical trouble. However, the meaning of "cpu.weight" is simple and intiutive: the total weight of the cgroup. But as explored above, there is deception in this simplicity. Prepare to add a few alternative methods for distributing weight. Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260605124051.338602724%40infradead.org --- kernel/sched/debug.c | 74 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 74 insertions(+) diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index 40584b27ea0c..507e4866ebb2 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -633,6 +633,76 @@ static void debugfs_fair_server_init(void) } } +#ifdef CONFIG_FAIR_GROUP_SCHED +static int cgroup_mode = 0; + +static const char *cgroup_mode_str[] = { + "smp", +}; + +static int sched_cgroup_mode(const char *str) +{ + for (int i = 0; i < ARRAY_SIZE(cgroup_mode_str); i++) { + if (!strcmp(str, cgroup_mode_str[i])) + return i; + } + return -EINVAL; +} + +static ssize_t sched_cgroup_write(struct file *filp, const char __user *ubuf, + size_t cnt, loff_t *ppos) +{ + char buf[16]; + int mode; + + if (cnt > 15) + cnt = 15; + + if (copy_from_user(buf, ubuf, cnt)) + return -EFAULT; + + buf[cnt] = 0; + mode = sched_cgroup_mode(strstrip(buf)); + if (mode < 0) + return mode; + + WRITE_ONCE(cgroup_mode, mode); + + *ppos += cnt; + return cnt; +} + +static int sched_cgroup_show(struct seq_file *m, void *v) +{ + int mode = READ_ONCE(cgroup_mode); + + for (int i = 0; i < ARRAY_SIZE(cgroup_mode_str); i++) { + if (mode == i) + seq_puts(m, "("); + seq_puts(m, cgroup_mode_str[i]); + if (mode == i) + seq_puts(m, ")"); + + seq_puts(m, " "); + } + seq_puts(m, "\n"); + return 0; +} + +static int sched_cgroup_open(struct inode *inode, struct file *filp) +{ + return single_open(filp, sched_cgroup_show, NULL); +} + +static const struct file_operations sched_cgroup_fops = { + .open = sched_cgroup_open, + .write = sched_cgroup_write, + .read = seq_read, + .llseek = seq_lseek, + .release = single_release, +}; +#endif + static __init int sched_init_debug(void) { struct dentry __maybe_unused *numa, *llc; @@ -686,6 +756,10 @@ static __init int sched_init_debug(void) debugfs_create_file("debug", 0444, debugfs_sched, NULL, &sched_debug_fops); +#ifdef CONFIG_FAIR_GROUP_SCHED + debugfs_create_file("cgroup_mode", 0644, debugfs_sched, NULL, &sched_cgroup_fops); +#endif + debugfs_fair_server_init(); #ifdef CONFIG_SCHED_CLASS_EXT debugfs_ext_server_init();