On Tue, Apr 16, 2024 at 2:38 PM Tejun Heo <tj@xxxxxxxxxx> wrote: > > On Tue, Apr 16, 2024 at 07:51:19PM +0200, Jesper Dangaard Brouer wrote: > > This patchset is focused on the global cgroup_rstat_lock. > > > > Patch-1: Adds tracepoints to improve measuring lock behavior. > > Patch-2: Converts the global lock into a mutex. > > Patch-3: Limits userspace triggered pressure on the lock. > > Imma wait for people's inputs on patch 2 and 3. ISTR switching the lock to > mutex made some tail latencies really bad for some workloads at google? > Yosry, was that you? I spent some time going through the history of my previous patchsets to find context. There were two separate instances where concerns were raised about using a mutex. (a) Converting the global rstat spinlock to a mutex: Shakeel had concerns about priority inversion with a global sleepable lock. So I never actually tested replacing the spinlock with a mutex based on Shakeel's concerns as priority inversions would be difficult to reproduce with synthetic tests. Generally speaking, other than priority inversions, I was depending on Wei's synthetic test to measure performance for userspace reads, and a script I wrote with parallel reclaimers to measure performance for in-kernel flushers. (b) Adding a mutex on top of the global rstat spinlock for userspace reads (to limit contention from userspace on the in-kernel lock): Wei reported that this significantly affects userspace read latency [2]. I then proceeded to add per-memcg thresholds for flushing, which resulted in the regressions from that mutex going away. However, at that point the mutex didn't really provide much value, so I removed it [3]. [1]https://lore.kernel.org/lkml/CALvZod441xBoXzhqLWTZ+xnqDOFkHmvrzspr9NAr+nybqXgS-A@xxxxxxxxxxxxxx/ [2]https://lore.kernel.org/lkml/CAAPL-u9D2b=iF5Lf_cRnKxUfkiEe0AMDTu6yhrUAzX0b6a6rDg@xxxxxxxxxxxxxx/ [3]https://lore.kernel.org/lkml/CAJD7tkZgP3m-VVPn+fF_YuvXeQYK=tZZjJHj=dzD=CcSSpp2qg@xxxxxxxxxxxxxx/
