mm: mmap_lock: add tracepoints around lock acquisition
The goal of these tracepoints is to be able to debug lock contention issues. This lock is acquired on most (all?) mmap / munmap / page fault operations, so a multi-threaded process which does a lot of these can experience significant contention. We trace just before we start acquisition, when the acquisition returns (whether it succeeded or not), and when the lock is released (or downgraded). The events are broken out by lock type (read / write). The events are also broken out by memcg path. For container-based workloads, users often think of several processes in a memcg as a single logical "task", so collecting statistics at this level is useful. The end goal is to get latency information. This isn't directly included in the trace events. Instead, users are expected to compute the time between "start locking" and "acquire returned", using e.g. synthetic events or BPF. The benefit we get from this is simpler code. Because we use tracepoint_...
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