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4cc39f91ef
Patch series "mm/madvise: remove redundant mmap_lock operations from
process_madvise()".
process_madvise() calls do_madvise() for each address range. Then, each
do_madvise() invocation holds and releases same mmap_lock. Optimize the
redundant lock operations by splitting do_madvise() internal logic
including the mmap_lock operations, and calling the small logic directly
from process_madvise() in a sequence that removes the redundant locking.
As a result of this change, process_madvise() becomes more efficient and
less racy in terms of its results and latency.
Note that the potential downside of this series is that other mmap_lock
holders may take more time due to the increased length of mmap_lock
critical section for process_madvise() calls. But there is maximum limit
in the kernel space (IOV_MAX), and userspace can control the critical
section length by setting the request size. Hence, the downside would be
limited and controllable.
Evaluation
==========
I measured the time to apply MADV_DONTNEED advice to 256 MiB memory using
multiple madvise() calls, 4 KiB per each call. I also do the same with
process_madvise(), but with varying batch size (vlen) from 1 to 1024. The
source code for the measurement is available at GitHub[1]. Because the
microbenchmark result is not that stable, I ran each configuration five
times and use the average.
The measurement results are as below. 'sz_batches' column shows the batch
size of process_madvise() calls. '0' batch size is for madvise() calls
case. 'before' and 'after' columns are the measured time to apply
MADV_DONTNEED to the 256 MiB memory buffer in nanoseconds, on kernels that
built without and with the last patch of this series, respectively. So
lower value means better efficiency. 'after/before' column is the ratio
of 'after' to 'before'.
sz_batches before after after/before
0 146294215.2 121280536.2 0.829017989769427
1 165851018.8 136305598.2 0.821855658085351
2 129469321.2 103740383.6 0.801273866569094
4 110369232.4 87835896.2 0.795836795182785
8 102906232.4 77420920.2 0.752344327397609
16 97551017.4 74959714.4 0.768415506038587
32 94809848.2 71200848.4 0.750985786305689
64 96087575.6 72593180 0.755489765942227
128 96154163.8 68517055.4 0.712575022154163
256 92901257.6 69054216.6 0.743307662177439
512 93646170.8 67053296.2 0.716028168874151
1024 92663219.2 70168196.8 0.75723892830177
Despite the unstable nature of the test program, the trend is as we
expect. The measurement shows this patchset reduces the process_madvise()
latency, proportional to the batching size. The latency gain was about
20% with the batch size 2, and it has increased to about 28% with the
batch size 512, since more number of mmap locking is reduced with larger
batch size.
Note that the standard devitation of the measurements for each sz_batches
configuration ranged from 1.9% to 7.2%. That is, this result is not very
stable. The average of the standard deviations for different batch sizes
were 4.62% and 4.70% for the 'before' and 'after' kernel measurements.
Also note that this patch has somehow decreased latencies of madvise() and
single batch size process_madvise(). Seems this code path is small enough
to significantly be affected by compiler optimizations including inlining
of split-out functions. Please focus on only the improvement amount that
changed by the batch size.
[1] https://github.com/sjp38/eval_proc_madvise
This patch (of 4):
Split out the madvise behavior-dependent mmap_lock operations from
do_madvise(), for easier reuse of the logic in an upcoming change.
[lorenzo.stoakes@oracle.com: fix madvise_[un]lock() issue]
Link: https://lkml.kernel.org/r/2f448f7b-1da7-4099-aa9e-0179d47fde40@lucifer.local
[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20250206061517.2958-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250206061517.2958-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Liam R. Howlett <howlett@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.