Re: [PATCH 3/4 v2] cgroup: separate rstat locks for subsystems

JP Kobryn
2025-03-06T21:47:49Z

On 2/28/25 11:20 AM, Yosry Ahmed wrote:
> On Thu, Feb 27, 2025 at 01:55:42PM -0800, inwardvessel wrote:
>> From: JP Kobryn <inwardvessel@gmail.com>
>>
>> Let the existing locks be dedicated to the base stats and rename them as
>> such. Also add new rstat locks for each enabled subsystem. When handling
>> cgroup subsystem states, distinguish between formal subsystems (memory,
>> io, etc) and the base stats subsystem state (represented by
>> cgroup::self) to decide on which locks to take. This change is made to
>> prevent contention between subsystems when updating/flushing stats.
>>
>> Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
>> ---
>> kernel/cgroup/rstat.c | 93 +++++++++++++++++++++++++++++++++----------
>> 1 file changed, 72 insertions(+), 21 deletions(-)
>>
>> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
>> index 88908ef9212d..b3eaefc1fd07 100644
>> --- a/kernel/cgroup/rstat.c
>> +++ b/kernel/cgroup/rstat.c
>> @@ -9,8 +9,12 @@
>>
>> #include <trace/events/cgroup.h>
>>
>> -static DEFINE_SPINLOCK(cgroup_rstat_lock);
>> -static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_cpu_lock);
>> +static DEFINE_SPINLOCK(cgroup_rstat_base_lock);
>> +static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_base_cpu_lock);
>> +
>> +static spinlock_t cgroup_rstat_subsys_lock[CGROUP_SUBSYS_COUNT];
>> +static DEFINE_PER_CPU(raw_spinlock_t,
>> + cgroup_rstat_subsys_cpu_lock[CGROUP_SUBSYS_COUNT]);
>>
>> static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu);
>>
>> @@ -20,8 +24,13 @@ static struct cgroup_rstat_cpu *cgroup_rstat_cpu(
>> return per_cpu_ptr(css->rstat_cpu, cpu);
>> }
>>
>> +static inline bool is_base_css(struct cgroup_subsys_state *css)
>> +{
>> + return css->ss == NULL;
>> +}
>> +
>> /*
>> - * Helper functions for rstat per CPU lock (cgroup_rstat_cpu_lock).
>> + * Helper functions for rstat per CPU locks.
>> *
>> * This makes it easier to diagnose locking issues and contention in
>> * production environments. The parameter @fast_path determine the
>> @@ -36,12 +45,12 @@ unsigned long _cgroup_rstat_cpu_lock(raw_spinlock_t *cpu_lock, int cpu,
>> bool contended;
>>
>> /*
>> - * The _irqsave() is needed because cgroup_rstat_lock is
>> - * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
>> - * this lock with the _irq() suffix only disables interrupts on
>> - * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
>> - * interrupts on both configurations. The _irqsave() ensures
>> - * that interrupts are always disabled and later restored.
>> + * The _irqsave() is needed because the locks used for flushing are
>> + * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring this lock
>> + * with the _irq() suffix only disables interrupts on a non-PREEMPT_RT
>> + * kernel. The raw_spinlock_t below disables interrupts on both
>> + * configurations. The _irqsave() ensures that interrupts are always
>> + * disabled and later restored.
>> */
>> contended = !raw_spin_trylock_irqsave(cpu_lock, flags);
>> if (contended) {
>> @@ -87,7 +96,7 @@ __bpf_kfunc void cgroup_rstat_updated(
>> struct cgroup_subsys_state *css, int cpu)
>> {
>> struct cgroup *cgrp = css->cgroup;
>> - raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
>> + raw_spinlock_t *cpu_lock;
>> unsigned long flags;
>>
>> /*
>> @@ -101,6 +110,12 @@ __bpf_kfunc void cgroup_rstat_updated(
>> if (data_race(cgroup_rstat_cpu(css, cpu)->updated_next))
>> return;
>>
>> + if (is_base_css(css))
>> + cpu_lock = per_cpu_ptr(&cgroup_rstat_base_cpu_lock, cpu);
>> + else
>> + cpu_lock = per_cpu_ptr(cgroup_rstat_subsys_cpu_lock, cpu) +
>> + css->ss->id;
>> +
>
> Maybe wrap this in a macro or function since it's used more than once.
>
>> flags = _cgroup_rstat_cpu_lock(cpu_lock, cpu, cgrp, true);
>>
>> /* put @css and all ancestors on the corresponding updated lists */
>> @@ -208,11 +223,17 @@ static struct cgroup_subsys_state *cgroup_rstat_updated_list(
>> struct cgroup_subsys_state *root, int cpu)
>> {
>> struct cgroup *cgrp = root->cgroup;
>> - raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
>> struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(root, cpu);
>> struct cgroup_subsys_state *head = NULL, *parent, *child;
>> + raw_spinlock_t *cpu_lock;
>> unsigned long flags;
>>
>> + if (is_base_css(root))
>> + cpu_lock = per_cpu_ptr(&cgroup_rstat_base_cpu_lock, cpu);
>> + else
>> + cpu_lock = per_cpu_ptr(cgroup_rstat_subsys_cpu_lock, cpu) +
>> + root->ss->id;
>> +
>> flags = _cgroup_rstat_cpu_lock(cpu_lock, cpu, cgrp, false);
>>
>> /* Return NULL if this subtree is not on-list */
>> @@ -315,7 +336,7 @@ static void cgroup_rstat_flush_locked(struct cgroup_subsys_state *css,
>> struct cgroup *cgrp = css->cgroup;
>> int cpu;
>>
>> - lockdep_assert_held(&cgroup_rstat_lock);
>> + lockdep_assert_held(&lock);
>>
>> for_each_possible_cpu(cpu) {
>> struct cgroup_subsys_state *pos;
>> @@ -356,12 +377,18 @@ static void cgroup_rstat_flush_locked(struct cgroup_subsys_state *css,
>> __bpf_kfunc void cgroup_rstat_flush(struct cgroup_subsys_state *css)
>> {
>> struct cgroup *cgrp = css->cgroup;
>> + spinlock_t *lock;
>> +
>> + if (is_base_css(css))
>> + lock = &cgroup_rstat_base_lock;
>> + else
>> + lock = &cgroup_rstat_subsys_lock[css->ss->id];
>
> Same here. Also, instead of passing locks around, can we just pass
> the css to __cgroup_rstat_lock/unlock() and have them find the correct
> lock and use it directly?

I like this and will take that approach in v3. It will allow the lock
selection code you previously commented on to exist in just one place.

>
>>
>> might_sleep();
>>
>> - __cgroup_rstat_lock(&cgroup_rstat_lock, cgrp, -1);
>> - cgroup_rstat_flush_locked(css, &cgroup_rstat_lock);
>> - __cgroup_rstat_unlock(&cgroup_rstat_lock, cgrp, -1);
>> + __cgroup_rstat_lock(lock, cgrp, -1);
>> + cgroup_rstat_flush_locked(css, lock);
>> + __cgroup_rstat_unlock(lock, cgrp, -1);
>> }
>>
>> /**
>> @@ -376,10 +403,16 @@ __bpf_kfunc void cgroup_rstat_flush(struct cgroup_subsys_state *css)
>> void cgroup_rstat_flush_hold(struct cgroup_subsys_state *css)
>> {
>> struct cgroup *cgrp = css->cgroup;
>> + spinlock_t *lock;
>> +
>> + if (is_base_css(css))
>> + lock = &cgroup_rstat_base_lock;
>> + else
>> + lock = &cgroup_rstat_subsys_lock[css->ss->id];
>>
>> might_sleep();
>> - __cgroup_rstat_lock(&cgroup_rstat_lock, cgrp, -1);
>> - cgroup_rstat_flush_locked(css, &cgroup_rstat_lock);
>> + __cgroup_rstat_lock(lock, cgrp, -1);
>> + cgroup_rstat_flush_locked(css, lock);
>> }
>>
>> /**
>> @@ -389,7 +422,14 @@ void cgroup_rstat_flush_hold(struct cgroup_subsys_state *css)
>> void cgroup_rstat_flush_release(struct cgroup_subsys_state *css)
>> {
>> struct cgroup *cgrp = css->cgroup;
>> - __cgroup_rstat_unlock(&cgroup_rstat_lock, cgrp, -1);
>> + spinlock_t *lock;
>> +
>> + if (is_base_css(css))
>> + lock = &cgroup_rstat_base_lock;
>> + else
>> + lock = &cgroup_rstat_subsys_lock[css->ss->id];
>> +
>> + __cgroup_rstat_unlock(lock, cgrp, -1);
>> }
>>
>> int cgroup_rstat_init(struct cgroup_subsys_state *css)
>> @@ -435,10 +475,21 @@ void cgroup_rstat_exit(struct cgroup_subsys_state *css)
>>
>> void __init cgroup_rstat_boot(void)
>> {
>> - int cpu;
>> + struct cgroup_subsys *ss;
>> + int cpu, ssid;
>>
>> - for_each_possible_cpu(cpu)
>> - raw_spin_lock_init(per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu));
>> + for_each_subsys(ss, ssid) {
>> + spin_lock_init(&cgroup_rstat_subsys_lock[ssid]);
>> + }
>> +
>> + for_each_possible_cpu(cpu) {
>> + raw_spin_lock_init(per_cpu_ptr(&cgroup_rstat_base_cpu_lock, cpu));
>> +
>> + for_each_subsys(ss, ssid) {
>> + raw_spin_lock_init(
>> + per_cpu_ptr(cgroup_rstat_subsys_cpu_lock, cpu) + ssid);
>> + }
>> + }
>> }
>>
>> /*
>> --
>> 2.43.5
>>
>>