Re: [PATCH v2 0/7] Add histogram measuring mmap_lock contention latency

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 28 May 2020 16:52:38 -0700
Axel Rasmussen <axelrasmussen@google.com> wrote:

Hi Axel,

First, your patch threading is messed up. All the patches should be a
reply to this cover page, and not individual emails which get lost
among other patches.

Next, we already have histogram logic with trace events. Why not build
off of that. Or perhaps update lockdep which can record contention with
all locks. Why create yet another histogram infrastructure that is used
for just a specific purpose?

-- Steve


> 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@google.com'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