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Patch series "Introduce __mt_dup() to improve the performance of fork()", v7.

This series introduces __mt_dup() to improve the performance of fork(). 
During the duplication process of mmap, all VMAs are traversed and
inserted one by one into the new maple tree, causing the maple tree to be
rebalanced multiple times.  Balancing the maple tree is a costly
operation.  To duplicate VMAs more efficiently, mtree_dup() and __mt_dup()
are introduced for the maple tree.  They can efficiently duplicate a maple
tree.

Here are some algorithmic details about {mtree,__mt}_dup().  We perform a
DFS pre-order traversal of all nodes in the source maple tree.  During
this process, we fully copy the nodes from the source tree to the new
tree.  This involves memory allocation, and when encountering a new node,
if it is a non-leaf node, all its child nodes are allocated at once.

This idea was originally from Liam R.  Howlett's Maple Tree Work email,
and I added some of my own ideas to implement it.  Some previous
discussions can be found in [1].  For a more detailed analysis of the
algorithm, please refer to the logs for patch [3/10] and patch [10/10].

There is a "spawn" in byte-unixbench[2], which can be used to test the
performance of fork().  I modified it slightly to make it work with
different number of VMAs.

Below are the test results.  The first row shows the number of VMAs.  The
second and third rows show the number of fork() calls per ten seconds,
corresponding to next-20231006 and the this patchset, respectively.  The
test results were obtained with CPU binding to avoid scheduler load
balancing that could cause unstable results.  There are still some
fluctuations in the test results, but at least they are better than the
original performance.

21     121   221    421    821    1621   3221   6421   12821  25621  51221
112100 76261 54227  34035  20195  11112  6017   3161   1606   802    393
114558 83067 65008  45824  28751  16072  8922   4747   2436   1233   599
2.19%  8.92% 19.88% 34.64% 42.37% 44.64% 48.28% 50.17% 51.68% 53.74% 52.42%

Thanks to Liam and Matthew for the review.


This patch (of 10):

Add two helpers:
1. mt_free_one(), used to free a maple node.
2. mt_attr(), used to obtain the attributes of maple tree.

Link: https://lkml.kernel.org/r/20231027033845.90608-1-zhangpeng.00@bytedance.com
Link: https://lkml.kernel.org/r/20231027033845.90608-2-zhangpeng.00@bytedance.com

Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>