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Message-ID: <30d64e25-561a-41c6-ab95-f0820248e9b6@redhat.com>
Date: Wed, 1 May 2024 10:24:04 -0400
From: Waiman Long <longman@...hat.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org,
hannes@...xchg.org, lizefan.x@...edance.com, cgroups@...r.kernel.org,
yosryahmed@...gle.com
Cc: netdev@...r.kernel.org, linux-mm@...ck.org, shakeel.butt@...ux.dev,
kernel-team@...udflare.com, Arnaldo Carvalho de Melo <acme@...nel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v1] cgroup/rstat: add cgroup_rstat_cpu_lock helpers and
tracepoints
On 5/1/24 10:04, Jesper Dangaard Brouer wrote:
> This closely resembles helpers added for the global cgroup_rstat_lock in
> commit fc29e04ae1ad ("cgroup/rstat: add cgroup_rstat_lock helpers and
> tracepoints"). This is for the per CPU lock cgroup_rstat_cpu_lock.
>
> Based on production workloads, we observe the fast-path "update" function
> cgroup_rstat_updated() is invoked around 3 million times per sec, while the
> "flush" function cgroup_rstat_flush_locked(), walking each possible CPU,
> can see periodic spikes of 700 invocations/sec.
>
> For this reason, the tracepoints are split into normal and fastpath
> versions for this per-CPU lock. Making it feasible for production to
> continuously monitor the non-fastpath tracepoint to detect lock contention
> issues. The reason for monitoring is that lock disables IRQs which can
> disturb e.g. softirq processing on the local CPUs involved. When the
> global cgroup_rstat_lock stops disabling IRQs (e.g converted to a mutex),
> this per CPU lock becomes the next bottleneck that can introduce latency
> variations.
>
> A practical bpftrace script for monitoring contention latency:
>
> bpftrace -e '
> tracepoint:cgroup:cgroup_rstat_cpu_lock_contended {
> @start[tid]=nsecs; @cnt[probe]=count()}
> tracepoint:cgroup:cgroup_rstat_cpu_locked {
> if (args->contended) {
> @wait_ns=hist(nsecs-@...rt[tid]); delete(@start[tid]);}
> @cnt[probe]=count()}
> interval:s:1 {time("%H:%M:%S "); print(@wait_ns); print(@cnt); clear(@cnt);}'
This is a per-cpu lock. So the only possible contention involves only 2
CPUs - a local CPU invoking cgroup_rstat_updated(). A flusher CPU doing
cgroup_rstat_flush_locked() calling into cgroup_rstat_updated_list().
With recent commits to reduce the percpu lock hold time, I doubt lock
contention on the percpu lock will have a great impact on latency. So do
we really need such an elaborate scheme to monitor this? BTW, the
additional code will also add to the worst case latency.
Cheers,
Longman
>
> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
> ---
> include/trace/events/cgroup.h | 56 +++++++++++++++++++++++++++++----
> kernel/cgroup/rstat.c | 70 ++++++++++++++++++++++++++++++++++-------
> 2 files changed, 108 insertions(+), 18 deletions(-)
>
> diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
> index 13f375800135..0b95865a90f3 100644
> --- a/include/trace/events/cgroup.h
> +++ b/include/trace/events/cgroup.h
> @@ -206,15 +206,15 @@ DEFINE_EVENT(cgroup_event, cgroup_notify_frozen,
>
> DECLARE_EVENT_CLASS(cgroup_rstat,
>
> - TP_PROTO(struct cgroup *cgrp, int cpu_in_loop, bool contended),
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
>
> - TP_ARGS(cgrp, cpu_in_loop, contended),
> + TP_ARGS(cgrp, cpu, contended),
>
> TP_STRUCT__entry(
> __field( int, root )
> __field( int, level )
> __field( u64, id )
> - __field( int, cpu_in_loop )
> + __field( int, cpu )
> __field( bool, contended )
> ),
>
> @@ -222,15 +222,16 @@ DECLARE_EVENT_CLASS(cgroup_rstat,
> __entry->root = cgrp->root->hierarchy_id;
> __entry->id = cgroup_id(cgrp);
> __entry->level = cgrp->level;
> - __entry->cpu_in_loop = cpu_in_loop;
> + __entry->cpu = cpu;
> __entry->contended = contended;
> ),
>
> - TP_printk("root=%d id=%llu level=%d cpu_in_loop=%d lock contended:%d",
> + TP_printk("root=%d id=%llu level=%d cpu=%d lock contended:%d",
> __entry->root, __entry->id, __entry->level,
> - __entry->cpu_in_loop, __entry->contended)
> + __entry->cpu, __entry->contended)
> );
>
> +/* Related to global: cgroup_rstat_lock */
> DEFINE_EVENT(cgroup_rstat, cgroup_rstat_lock_contended,
>
> TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> @@ -252,6 +253,49 @@ DEFINE_EVENT(cgroup_rstat, cgroup_rstat_unlock,
> TP_ARGS(cgrp, cpu, contended)
> );
>
> +/* Related to per CPU: cgroup_rstat_cpu_lock */
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_lock_contended,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_lock_contended_fastpath,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_locked,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_locked_fastpath,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_unlock,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> +DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_unlock_fastpath,
> +
> + TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
> +
> + TP_ARGS(cgrp, cpu, contended)
> +);
> +
> #endif /* _TRACE_CGROUP_H */
>
> /* This part must be outside protection */
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index 52e3b0ed1cee..fb8b49437573 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -19,6 +19,60 @@ static struct cgroup_rstat_cpu *cgroup_rstat_cpu(struct cgroup *cgrp, int cpu)
> return per_cpu_ptr(cgrp->rstat_cpu, cpu);
> }
>
> +/*
> + * Helper functions for rstat per CPU lock (cgroup_rstat_cpu_lock).
> + *
> + * This makes it easier to diagnose locking issues and contention in
> + * production environments. The parameter @fast_path determine the
> + * tracepoints being added, allowing us to diagnose "flush" related
> + * operations without handling high-frequency fast-path "update" events.
> + */
> +static __always_inline
> +unsigned long _cgroup_rstat_cpu_lock(raw_spinlock_t *cpu_lock, int cpu,
> + struct cgroup *cgrp, const bool fast_path)
> +{
> + unsigned long flags;
> + 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.
> + */
> + contended = !raw_spin_trylock_irqsave(cpu_lock, flags);
> + if (contended) {
> + if (fast_path)
> + trace_cgroup_rstat_cpu_lock_contended_fastpath(cgrp, cpu, contended);
> + else
> + trace_cgroup_rstat_cpu_lock_contended(cgrp, cpu, contended);
> +
> + raw_spin_lock_irqsave(cpu_lock, flags);
> + }
> +
> + if (fast_path)
> + trace_cgroup_rstat_cpu_locked_fastpath(cgrp, cpu, contended);
> + else
> + trace_cgroup_rstat_cpu_locked(cgrp, cpu, contended);
> +
> + return flags;
> +}
> +
> +static __always_inline
> +void _cgroup_rstat_cpu_unlock(raw_spinlock_t *cpu_lock, int cpu,
> + struct cgroup *cgrp, unsigned long flags,
> + const bool fast_path)
> +{
> + if (fast_path)
> + trace_cgroup_rstat_cpu_unlock_fastpath(cgrp, cpu, false);
> + else
> + trace_cgroup_rstat_cpu_unlock(cgrp, cpu, false);
> +
> + raw_spin_unlock_irqrestore(cpu_lock, flags);
> +}
> +
> /**
> * cgroup_rstat_updated - keep track of updated rstat_cpu
> * @cgrp: target cgroup
> @@ -44,7 +98,7 @@ __bpf_kfunc void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
> if (data_race(cgroup_rstat_cpu(cgrp, cpu)->updated_next))
> return;
>
> - raw_spin_lock_irqsave(cpu_lock, flags);
> + flags = _cgroup_rstat_cpu_lock(cpu_lock, cpu, cgrp, true);
>
> /* put @cgrp and all ancestors on the corresponding updated lists */
> while (true) {
> @@ -72,7 +126,7 @@ __bpf_kfunc void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
> cgrp = parent;
> }
>
> - raw_spin_unlock_irqrestore(cpu_lock, flags);
> + _cgroup_rstat_cpu_unlock(cpu_lock, cpu, cgrp, flags, true);
> }
>
> /**
> @@ -153,15 +207,7 @@ static struct cgroup *cgroup_rstat_updated_list(struct cgroup *root, int cpu)
> struct cgroup *head = NULL, *parent, *child;
> unsigned long flags;
>
> - /*
> - * 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.
> - */
> - raw_spin_lock_irqsave(cpu_lock, flags);
> + flags = _cgroup_rstat_cpu_lock(cpu_lock, cpu, root, false);
>
> /* Return NULL if this subtree is not on-list */
> if (!rstatc->updated_next)
> @@ -198,7 +244,7 @@ static struct cgroup *cgroup_rstat_updated_list(struct cgroup *root, int cpu)
> if (child != root)
> head = cgroup_rstat_push_children(head, child, cpu);
> unlock_ret:
> - raw_spin_unlock_irqrestore(cpu_lock, flags);
> + _cgroup_rstat_cpu_unlock(cpu_lock, cpu, root, flags, false);
> return head;
> }
>
>
>
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