Messages in this thread | | | From | Yosry Ahmed <> | Date | Wed, 17 Apr 2024 19:13:40 -0700 | Subject | Re: [PATCH v1 0/3] cgroup/rstat: global cgroup_rstat_lock changes |
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On Tue, Apr 16, 2024 at 2:38 PM Tejun Heo <tj@kernel.org> 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@mail.gmail.com/ [2]https://lore.kernel.org/lkml/CAAPL-u9D2b=iF5Lf_cRnKxUfkiEe0AMDTu6yhrUAzX0b6a6rDg@mail.gmail.com/ [3]https://lore.kernel.org/lkml/CAJD7tkZgP3m-VVPn+fF_YuvXeQYK=tZZjJHj=dzD=CcSSpp2qg@mail.gmail.com/
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