Re: [PATCH 01/11] cgroup: move rstat pointers into struct of their own

Yosry Ahmed
2025-02-20T16:53:51Z

On Mon, Feb 17, 2025 at 07:14:38PM -0800, JP Kobryn wrote:
> The rstat infrastructure makes use of pointers for list management.
> These pointers only exist as fields in the cgroup struct, so moving them
> into their own struct will allow them to be used elsewhere. The base
> stat entities are included with them for now.
>
> Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
> ---
> include/linux/cgroup-defs.h | 90 +-----------------
> include/linux/cgroup_rstat.h | 92 +++++++++++++++++++
> kernel/cgroup/cgroup.c | 3 +-
> kernel/cgroup/rstat.c | 27 +++---
> .../selftests/bpf/progs/btf_type_tag_percpu.c | 4 +-
> 5 files changed, 112 insertions(+), 104 deletions(-)
> create mode 100644 include/linux/cgroup_rstat.h
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 1b20d2d8ef7c..6b6cc027fe70 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -17,7 +17,7 @@
> #include <linux/refcount.h>
> #include <linux/percpu-refcount.h>
> #include <linux/percpu-rwsem.h>
> -#include <linux/u64_stats_sync.h>
> +#include <linux/cgroup_rstat.h>
> #include <linux/workqueue.h>
> #include <linux/bpf-cgroup-defs.h>
> #include <linux/psi_types.h>
> @@ -321,78 +321,6 @@ struct css_set {
> struct rcu_head rcu_head;
> };
>
> -struct cgroup_base_stat {
> - struct task_cputime cputime;
> -
> -#ifdef CONFIG_SCHED_CORE
> - u64 forceidle_sum;
> -#endif
> - u64 ntime;
> -};
> -
> -/*
> - * rstat - cgroup scalable recursive statistics. Accounting is done
> - * per-cpu in cgroup_rstat_cpu which is then lazily propagated up the
> - * hierarchy on reads.
> - *
> - * When a stat gets updated, the cgroup_rstat_cpu and its ancestors are
> - * linked into the updated tree. On the following read, propagation only
> - * considers and consumes the updated tree. This makes reading O(the
> - * number of descendants which have been active since last read) instead of
> - * O(the total number of descendants).
> - *
> - * This is important because there can be a lot of (draining) cgroups which
> - * aren't active and stat may be read frequently. The combination can
> - * become very expensive. By propagating selectively, increasing reading
> - * frequency decreases the cost of each read.
> - *
> - * This struct hosts both the fields which implement the above -
> - * updated_children and updated_next - and the fields which track basic
> - * resource statistics on top of it - bsync, bstat and last_bstat.
> - */
> -struct cgroup_rstat_cpu {
> - /*
> - * ->bsync protects ->bstat. These are the only fields which get
> - * updated in the hot path.
> - */
> - struct u64_stats_sync bsync;
> - struct cgroup_base_stat bstat;
> -
> - /*
> - * Snapshots at the last reading. These are used to calculate the
> - * deltas to propagate to the global counters.
> - */
> - struct cgroup_base_stat last_bstat;
> -
> - /*
> - * This field is used to record the cumulative per-cpu time of
> - * the cgroup and its descendants. Currently it can be read via
> - * eBPF/drgn etc, and we are still trying to determine how to
> - * expose it in the cgroupfs interface.
> - */
> - struct cgroup_base_stat subtree_bstat;
> -
> - /*
> - * Snapshots at the last reading. These are used to calculate the
> - * deltas to propagate to the per-cpu subtree_bstat.
> - */
> - struct cgroup_base_stat last_subtree_bstat;
> -
> - /*
> - * Child cgroups with stat updates on this cpu since the last read
> - * are linked on the parent's ->updated_children through
> - * ->updated_next.
> - *
> - * In addition to being more compact, singly-linked list pointing
> - * to the cgroup makes it unnecessary for each per-cpu struct to
> - * point back to the associated cgroup.
> - *
> - * Protected by per-cpu cgroup_rstat_cpu_lock.
> - */
> - struct cgroup *updated_children; /* terminated by self cgroup */
> - struct cgroup *updated_next; /* NULL iff not on the list */
> -};
> -
> struct cgroup_freezer_state {
> /* Should the cgroup and its descendants be frozen. */
> bool freeze;
> @@ -517,23 +445,9 @@ struct cgroup {
> struct cgroup *old_dom_cgrp; /* used while enabling threaded */
>
> /* per-cpu recursive resource statistics */
> - struct cgroup_rstat_cpu __percpu *rstat_cpu;
> + struct cgroup_rstat rstat;
> struct list_head rstat_css_list;
>
> - /*
> - * Add padding to separate the read mostly rstat_cpu and
> - * rstat_css_list into a different cacheline from the following
> - * rstat_flush_next and *bstat fields which can have frequent updates.
> - */
> - CACHELINE_PADDING(_pad_);
> -
> - /*
> - * A singly-linked list of cgroup structures to be rstat flushed.
> - * This is a scratch field to be used exclusively by
> - * cgroup_rstat_flush_locked() and protected by cgroup_rstat_lock.
> - */
> - struct cgroup *rstat_flush_next;
> -
> /* cgroup basic resource statistics */
> struct cgroup_base_stat last_bstat;
> struct cgroup_base_stat bstat;
> diff --git a/include/linux/cgroup_rstat.h b/include/linux/cgroup_rstat.h
> new file mode 100644
> index 000000000000..f95474d6f8ab
> --- /dev/null
> +++ b/include/linux/cgroup_rstat.h
> @@ -0,0 +1,92 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_RSTAT_H
> +#define _LINUX_RSTAT_H
> +
> +#include <linux/u64_stats_sync.h>
> +
> +struct cgroup_rstat_cpu;

Why do we need the forward declaration instead of just defining struct
cgroup_rstat_cpu first? Also, why do we need a new header for these
definitions rather than just adding struct cgroup_rstat to
cgroup-defs.h?

> +
> +/*
> + * rstat - cgroup scalable recursive statistics. Accounting is done
> + * per-cpu in cgroup_rstat_cpu which is then lazily propagated up the
> + * hierarchy on reads.
> + *
> + * When a stat gets updated, the cgroup_rstat_cpu and its ancestors are
> + * linked into the updated tree. On the following read, propagation only
> + * considers and consumes the updated tree. This makes reading O(the
> + * number of descendants which have been active since last read) instead of
> + * O(the total number of descendants).
> + *
> + * This is important because there can be a lot of (draining) cgroups which
> + * aren't active and stat may be read frequently. The combination can
> + * become very expensive. By propagating selectively, increasing reading
> + * frequency decreases the cost of each read.
> + *
> + * This struct hosts both the fields which implement the above -
> + * updated_children and updated_next - and the fields which track basic
> + * resource statistics on top of it - bsync, bstat and last_bstat.
> + */
> +struct cgroup_rstat {
> + struct cgroup_rstat_cpu __percpu *rstat_cpu;
> +
> + /*
> + * Add padding to separate the read mostly rstat_cpu and
> + * rstat_css_list into a different cacheline from the following
> + * rstat_flush_next and containing struct fields which can have
> + * frequent updates.
> + */
> + CACHELINE_PADDING(_pad_);
> + struct cgroup *rstat_flush_next;
> +};
> +
> +struct cgroup_base_stat {
> + struct task_cputime cputime;
> +
> +#ifdef CONFIG_SCHED_CORE
> + u64 forceidle_sum;
> +#endif
> + u64 ntime;
> +};
> +
> +struct cgroup_rstat_cpu {
> + /*
> + * Child cgroups with stat updates on this cpu since the last read
> + * are linked on the parent's ->updated_children through
> + * ->updated_next.
> + *
> + * In addition to being more compact, singly-linked list pointing
> + * to the cgroup makes it unnecessary for each per-cpu struct to
> + * point back to the associated cgroup.
> + */
> + struct cgroup *updated_children; /* terminated by self */
> + struct cgroup *updated_next; /* NULL if not on the list */
> +
> + /*
> + * ->bsync protects ->bstat. These are the only fields which get
> + * updated in the hot path.
> + */
> + struct u64_stats_sync bsync;
> + struct cgroup_base_stat bstat;
> +
> + /*
> + * Snapshots at the last reading. These are used to calculate the
> + * deltas to propagate to the global counters.
> + */
> + struct cgroup_base_stat last_bstat;
> +
> + /*
> + * This field is used to record the cumulative per-cpu time of
> + * the cgroup and its descendants. Currently it can be read via
> + * eBPF/drgn etc, and we are still trying to determine how to
> + * expose it in the cgroupfs interface.
> + */
> + struct cgroup_base_stat subtree_bstat;
> +
> + /*
> + * Snapshots at the last reading. These are used to calculate the
> + * deltas to propagate to the per-cpu subtree_bstat.
> + */
> + struct cgroup_base_stat last_subtree_bstat;
> +};
> +
> +#endif /* _LINUX_RSTAT_H */