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Message-ID: <a309c2b5-5425-428c-a034-d5ebc68cb304@huaweicloud.com>
Date: Fri, 22 Aug 2025 14:58:48 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Tiffany Yang <ynaffit@...gle.com>, linux-kernel@...r.kernel.org
Cc: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
Stephen Boyd <sboyd@...nel.org>,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Michal Koutný
<mkoutny@...e.com>, "Rafael J. Wysocki" <rafael@...nel.org>,
Pavel Machek <pavel@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
Chen Ridong <chenridong@...wei.com>, kernel-team@...roid.com,
Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 1/2] cgroup: cgroup.stat.local time accounting
On 2025/8/22 14:14, Chen Ridong wrote:
>
>
> On 2025/8/22 9:37, Tiffany Yang wrote:
>> There isn't yet a clear way to identify a set of "lost" time that
>> everyone (or at least a wider group of users) cares about. However,
>> users can perform some delay accounting by iterating over components of
>> interest. This patch allows cgroup v2 freezing time to be one of those
>> components.
>>
>> Track the cumulative time that each v2 cgroup spends freezing and expose
>> it to userland via a new local stat file in cgroupfs. Thank you to
>> Michal, who provided the ASCII art in the updated documentation.
>>
>> To access this value:
>> $ mkdir /sys/fs/cgroup/test
>> $ cat /sys/fs/cgroup/test/cgroup.stat.local
>> freeze_time_total 0
>>
>> Ensure consistent freeze time reads with freeze_seq, a per-cgroup
>> sequence counter. Writes are serialized using the css_set_lock.
>>
>> Signed-off-by: Tiffany Yang <ynaffit@...gle.com>
>> Cc: Tejun Heo <tj@...nel.org>
>> Cc: Michal Koutný <mkoutny@...e.com>
>> ---
>> v3 -> v4:
>> * Replace "freeze_time_total" with "frozen" and expose stats via
>> cgroup.stat.local, as recommended by Tejun.
>> * Use the same timestamp when freezing/unfreezing a cgroup as its
>> descendants, as suggested by Michal.
>>
>> v2 -> v3:
>> * Use seqcount along with css_set_lock to guard freeze time accesses, as
>> suggested by Michal.
>>
>> v1 -> v2:
>> * Track per-cgroup freezing time instead of per-task frozen time, as
>> suggested by Tejun.
>> ---
>> Documentation/admin-guide/cgroup-v2.rst | 18 ++++++++++++++++
>> include/linux/cgroup-defs.h | 17 +++++++++++++++
>> kernel/cgroup/cgroup.c | 28 +++++++++++++++++++++++++
>> kernel/cgroup/freezer.c | 16 ++++++++++----
>> 4 files changed, 75 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>> index 51c0bc4c2dc5..a1e3d431974c 100644
>> --- a/Documentation/admin-guide/cgroup-v2.rst
>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>> @@ -1001,6 +1001,24 @@ All cgroup core files are prefixed with "cgroup."
>> Total number of dying cgroup subsystems (e.g. memory
>> cgroup) at and beneath the current cgroup.
>>
>> + cgroup.stat.local
>> + A read-only flat-keyed file which exists in non-root cgroups.
>> + The following entry is defined:
>> +
>> + frozen_usec
>> + Cumulative time that this cgroup has spent between freezing and
>> + thawing, regardless of whether by self or ancestor groups.
>> + NB: (not) reaching "frozen" state is not accounted here.
>> +
>> + Using the following ASCII representation of a cgroup's freezer
>> + state, ::
>> +
>> + 1 _____
>> + frozen 0 __/ \__
>> + ab cd
>> +
>> + the duration being measured is the span between a and c.
>> +
>> cgroup.freeze
>> A read-write single value file which exists on non-root cgroups.
>> Allowed values are "0" and "1". The default is "0".
>> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
>> index 6b93a64115fe..539c64eeef38 100644
>> --- a/include/linux/cgroup-defs.h
>> +++ b/include/linux/cgroup-defs.h
>> @@ -433,6 +433,23 @@ struct cgroup_freezer_state {
>> * frozen, SIGSTOPped, and PTRACEd.
>> */
>> int nr_frozen_tasks;
>> +
>> + /* Freeze time data consistency protection */
>> + seqcount_t freeze_seq;
>> +
>> + /*
>> + * Most recent time the cgroup was requested to freeze.
>> + * Accesses guarded by freeze_seq counter. Writes serialized
>> + * by css_set_lock.
>> + */
>> + u64 freeze_start_nsec;
>> +
>> + /*
>> + * Total duration the cgroup has spent freezing.
>> + * Accesses guarded by freeze_seq counter. Writes serialized
>> + * by css_set_lock.
>> + */
>> + u64 frozen_nsec;
>> };
>>
>> struct cgroup {
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index 312c6a8b55bb..ab096b884bbc 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -3763,6 +3763,27 @@ static int cgroup_stat_show(struct seq_file *seq, void *v)
>> return 0;
>> }
>>
>> +static int cgroup_core_local_stat_show(struct seq_file *seq, void *v)
>> +{
>> + struct cgroup *cgrp = seq_css(seq)->cgroup;
>> + unsigned int sequence;
>> + u64 freeze_time;
>> +
>> + do {
>> + sequence = read_seqcount_begin(&cgrp->freezer.freeze_seq);
>> + freeze_time = cgrp->freezer.frozen_nsec;
>> + /* Add in current freezer interval if the cgroup is freezing. */
>> + if (test_bit(CGRP_FREEZE, &cgrp->flags))
>> + freeze_time += (ktime_get_ns() -
>> + cgrp->freezer.freeze_start_nsec);
>> + } while (read_seqcount_retry(&cgrp->freezer.freeze_seq, sequence));
>> +
>> + seq_printf(seq, "frozen_usec %llu\n",
>> + (unsigned long long) freeze_time / NSEC_PER_USEC);
>> +
>> + return 0;
>> +}
>> +
>> #ifdef CONFIG_CGROUP_SCHED
>> /**
>> * cgroup_tryget_css - try to get a cgroup's css for the specified subsystem
>> @@ -5354,6 +5375,11 @@ static struct cftype cgroup_base_files[] = {
>> .name = "cgroup.stat",
>> .seq_show = cgroup_stat_show,
>> },
>> + {
>> + .name = "cgroup.stat.local",
>> + .flags = CFTYPE_NOT_ON_ROOT,
>> + .seq_show = cgroup_core_local_stat_show,
>> + },
>> {
>> .name = "cgroup.freeze",
>> .flags = CFTYPE_NOT_ON_ROOT,
>> @@ -5763,6 +5789,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
>> * if the parent has to be frozen, the child has too.
>> */
>> cgrp->freezer.e_freeze = parent->freezer.e_freeze;
>> + seqcount_init(&cgrp->freezer.freeze_seq);
>> if (cgrp->freezer.e_freeze) {
>> /*
>> * Set the CGRP_FREEZE flag, so when a process will be
>> @@ -5771,6 +5798,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
>> * consider it frozen immediately.
>> */
>> set_bit(CGRP_FREEZE, &cgrp->flags);
>> + cgrp->freezer.freeze_start_nsec = ktime_get_ns();
>> set_bit(CGRP_FROZEN, &cgrp->flags);
>> }
>>
>> diff --git a/kernel/cgroup/freezer.c b/kernel/cgroup/freezer.c
>> index bf1690a167dd..6c18854bff34 100644
>> --- a/kernel/cgroup/freezer.c
>> +++ b/kernel/cgroup/freezer.c
>> @@ -171,7 +171,7 @@ static void cgroup_freeze_task(struct task_struct *task, bool freeze)
>> /*
>> * Freeze or unfreeze all tasks in the given cgroup.
>> */
>> -static void cgroup_do_freeze(struct cgroup *cgrp, bool freeze)
>> +static void cgroup_do_freeze(struct cgroup *cgrp, bool freeze, u64 ts_nsec)
>> {
>> struct css_task_iter it;
>> struct task_struct *task;
>> @@ -179,10 +179,16 @@ static void cgroup_do_freeze(struct cgroup *cgrp, bool freeze)
>> lockdep_assert_held(&cgroup_mutex);
>>
>> spin_lock_irq(&css_set_lock);
>> - if (freeze)
>> + write_seqcount_begin(&cgrp->freezer.freeze_seq);
>> + if (freeze) {
>> set_bit(CGRP_FREEZE, &cgrp->flags);
>> - else
>> + cgrp->freezer.freeze_start_nsec = ts_nsec;
>> + } else {
>> clear_bit(CGRP_FREEZE, &cgrp->flags);
>> + cgrp->freezer.frozen_nsec += (ts_nsec -
>> + cgrp->freezer.freeze_start_nsec);
>> + }
>> + write_seqcount_end(&cgrp->freezer.freeze_seq);
>> spin_unlock_irq(&css_set_lock);
>>
>
> Hello Tiffany,
>
> I wanted to check if there are any specific considerations regarding how we should input the ts_nsec
> value.
>
> Would it be possible to define this directly within the cgroup_do_freeze function rather than
> passing it as a parameter? This approach might simplify the implementation and potentially improve
> timing accuracy when it have lots of descendants.
>
I revisited v3, and this was Michal's point.
p
/ | \
1 ... n
When we freeze the parent group p, is it expected that all descendant cgroups (1 to n) should share
the same frozen timestamp?
If the cgroup tree structure is stable, the exact frozen time may not be really matter. However, if
the tree is not stable, obtaining the same frozen time is acceptable?
--
Best regards,
Ridong
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