On 01/05/2024 20.41, Waiman Long wrote:
On 5/1/24 13:22, Jesper Dangaard Brouer wrote:
On 01/05/2024 16.24, Waiman Long wrote:
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-@start[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.
I do appriciate your recent changes to reduce the percpu lock hold time.
These tracepoints allow me to measure and differentiate the percpu lock
hold time vs. the flush time.
In production (using [1]) I'm seeing "Long lock-hold time" [L100] e.g.
upto 29 ms, which is time spend after obtaining the lock (runtime under
lock). I was expecting to see "High Lock-contention wait" [L82] which
is the time waiting for obtaining the lock.
This is why I'm adding these tracepoints, as they allow me to digg
deeper, to understand where this high runtime variations originate from.
Data:
16:52:09 Long lock-hold time: 14950 usec (14 ms) on CPU:34 comm:kswapd4
16:52:09 Long lock-hold time: 14821 usec (14 ms) on CPU:34 comm:kswapd4
16:52:09 Long lock-hold time: 11299 usec (11 ms) on CPU:98 comm:kswapd4
16:52:09 Long lock-hold time: 17237 usec (17 ms) on CPU:113 comm:kswapd6
16:52:09 Long lock-hold time: 29000 usec (29 ms) on CPU:36
comm:kworker/u261:12
That lock hold time is much higher than I would have expected.
16:52:09 time elapsed: 80 sec (interval = 1 sec)
Flushes(5033) 294/interval (avg 62/sec)
Locks(53374) 1748/interval (avg 667/sec)
Yields(48341) 1454/interval (avg 604/sec)
Contended(48104) 1450/interval (avg 601/sec)
So do we really need such an elaborate scheme to monitor this? BTW,
the additional code will also add to the worst case latency.
Hmm, I designed this code to have minimal impact, as tracepoints are
no-ops until activated. I really doubt this code will change the
latency.
[1]
https://github.com/xdp-project/xdp-project/blob/master/areas/latency/cgroup_rstat_tracepoint.bt
[L100]
https://github.com/xdp-project/xdp-project/blob/master/areas/latency/cgroup_rstat_tracepoint.bt#L100
[L82]
https://github.com/xdp-project/xdp-project/blob/master/areas/latency/cgroup_rstat_tracepoint.bt#L82
Signed-off-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
More data, the histogram of time spend under the lock have some strange
variation issues with a group in 4ms to 65ms area. Investigating what
can be causeing this... which next step depend in these tracepoints.
@lock_cnt: 759146
@locked_ns:
[1K, 2K) 499 | |
[2K, 4K) 206928
|@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4K, 8K) 147904 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[8K, 16K) 64453 |@@@@@@@@@@@@@@@@ |
[16K, 32K) 135467 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[32K, 64K) 75943 |@@@@@@@@@@@@@@@@@@@ |
[64K, 128K) 38359 |@@@@@@@@@ |
[128K, 256K) 46597 |@@@@@@@@@@@ |
[256K, 512K) 32466 |@@@@@@@@ |
[512K, 1M) 3945 | |
[1M, 2M) 642 | |
[2M, 4M) 750 | |
[4M, 8M) 1932 | |
[8M, 16M) 2114 | |
[16M, 32M) 1039 | |
[32M, 64M) 108 | |
---
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 >>>> +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);
Could you do a local_irq_save() before calling trace_cgroup*() and
raw_spin_lock()? Would that help in eliminating this high lock hold time?
Nope it will not eliminating high lock *hold* time, because the hold
start timestamp is first taken *AFTER* obtaining the lock.
It could help the contended "wait-time" measurement, but my prod
measurements show this isn't an issues.
You can also do a local_irq_save() first before the trylock. That will
eliminate the duplicated irq_restore() and irq_save() when there is
contention.
I wrote the code like this on purpose ;-)
My issue with this code/lock is it cause latency issues for softirq
NET_RX. So, when I detect a "contended" lock event, I do want a
irq_restore() as that will allow networking/do_softirq() to run before I
start waiting for the lock (with IRQ disabled).
If not, there may be NMIs mixed in.
NMIs are definitely on my list of things to investigate.
These AMD CPUs also have other types of interrupts that needs a close look.
The easier explaination is that the lock isn't "yielded" on every cycle
through the for each CPU loop.
Lets look at the data I provided above:
>> Flushes(5033) 294/interval (avg 62/sec)
>> Locks(53374) 1748/interval (avg 667/sec)
>> Yields(48341) 1454/interval (avg 604/sec)
>> Contended(48104) 1450/interval (avg 601/sec)
In this 1 second sample, we have 294 flushes, and more yields 1454,
great but the factor is not 128 (num-of-CPUs) but closer to 5. Thus, on
average we hold the lock for (128/5) 25.6 CPUs-walks.
We have spoken about releasing the lock on for_each CPU before... it
will likely solve this long hold time, but IMHO a mutex is still the
better solution.
--Jesper
