The overall goal of this patchset is to add a latency histogram which measures `mmap_lock` acquisition time. This is useful to measure the impact of ongoing work like maple trees and range locks (https://lwn.net/Articles/787629/), and it is also useful to debug userspace processes which experience long waits due to lock contention. This patchset is built upon walken@xxxxxxxxxx's new `mmap_lock` API (https://lkml.org/lkml/2020/4/21/1307). In its current form, it should apply cleanly to a 5.7-rc7 tree to which Michel's patchset has already been applied. To summarize the changes being made at a high level: - Add a histogram library: a `struct histogram` is effectively an array of thresholds (i.e., buckets), and an array of per-cpu `u64` counts of the number of samples in each bucket. - Modify Michel's mmap_lock API to record samples in a histogram, owned by the `mm_struct`, on each lock acquisition. For contended lock acquisitions, we compute the amount of time spent waiting, which determines the bucket. - For uncontended cases, we still record a sample, but with "0" latency. The reasoning for this is, a) we don't want to incur the overhead of actually measuring the time, but b) we still want to end up with an accurate count of acquisition attempts, as this lets us compute latency percentiles (e.g., "x% of lock acquisitions completed in <= y ns"). Changes since v1 (sent to a few folks within Google for initial review): - Added a tracepoint to the contended case. - Modified `mmap_write_lock_nested` to split the {un,}contended cases. - Removed support for having more than one histogram in `mm_struct`. - Removed any histogram code not explicitly used in this patchset. - Whitespace cleanups. Axel Rasmussen (7): histogram: add struct histogram histogram: add helper function to expose histograms to userspace mmap_lock: add a histogram structure to struct mm_struct mmap_lock: allocate histogram (if enabled) in mm_init mmap_lock: add /proc/<pid>/mmap_lock_contention interface mmap_lock: increment histogram whenever mmap_lock is acquired mmap_lock: add a tracepoint to contended acquisitions fs/proc/base.c | 25 +++ include/linux/histogram.h | 293 +++++++++++++++++++++++++++++++ include/linux/mm_types.h | 11 ++ include/linux/mmap_lock.h | 92 +++++++++- include/trace/events/mmap_lock.h | 34 ++++ kernel/fork.c | 55 ++++++ kernel/locking/rwsem.c | 4 +- lib/Kconfig | 3 + lib/Makefile | 2 + lib/histogram.c | 212 ++++++++++++++++++++++ mm/Kconfig | 13 ++ mm/Makefile | 1 + mm/mmap_lock.c | 46 +++++ 13 files changed, 782 insertions(+), 9 deletions(-) create mode 100644 include/linux/histogram.h create mode 100644 include/trace/events/mmap_lock.h create mode 100644 lib/histogram.c create mode 100644 mm/mmap_lock.c -- 2.27.0.rc0.183.gde8f92d652-goog
