On 17/07/2024 02.35, Yosry Ahmed wrote:
[..]This is a clean (meaning no cadvisor interference) example of kswapd starting simultaniously on many NUMA nodes, that in 27 out of 98 cases hit the race (which is handled in V6 and V7). The BPF "cnt" maps are getting cleared every second, so this approximates per sec numbers. This patch reduce pressure on the lock, but we are still seeing (kfunc:vmlinux:cgroup_rstat_flush_locked) full flushes approx 37 per sec (every 27 ms). On the positive side ongoing_flusher mitigation stopped 98 per sec of these. In this clean kswapd case the patch removes the lock contention issue for kswapd. The lock_contended cases 27 seems to be all related to handled_race cases 27. The remaning high flush rate should also be addressed, and we should also work on aproaches to limit this like my ealier proposal[1].I honestly don't think a high number of flushes is a problem on its own as long as we are not spending too much time flushing, especially when we have magnitude-based thresholding so we know there is something to flush (although it may not be relevant to what we are doing).
We are "spending too much time flushing" see below.
If we keep observing a lot of lock contention, one thing that I thought about is to have a variant of spin_lock with a timeout. This limits the flushing latency, instead of limiting the number of flushes (which I believe is the wrong metric to optimize). It also seems to me that we are doing a flush each 27ms, and your proposed threshold was once per 50ms. It doesn't seem like a fundamental difference.
Looking at the production numbers for the time the lock is held for level 0: @locked_time_level[0]: [4M, 8M) 623 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | [8M, 16M) 860 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [16M, 32M) 295 |@@@@@@@@@@@@@@@@@ | [32M, 64M) 275 |@@@@@@@@@@@@@@@@ | The time is in nanosec, so M corresponds to ms (milliseconds). With 36 flushes per second (as shown earlier) this is a flush every 27.7ms. It is not unreasonable (from above data) that the flush time also spend 27ms, which means that we spend a full CPU second flushing. That is spending too much time flushing. This around 1 sec CPU usage for kswapd is also quite clear in the attached grafana graph for when server was rebooted into this V7 kernel. I choose 50ms because at the time I saw flush taking around 30ms, and I view the flush time as queue service-time. When arrival-rate is faster than service-time, then a queue will form. So, choosing 50ms as arrival-rate gave me some headroom. As I mentioned earlier, optimally this threshold should be dynamically measured. --Jesper
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