Punit Agrawal [off-list ref] writes:

Suren Baghdasaryan [off-list ref] writes:

quoted

Previous version:
v1: https://lore.kernel.org/all/20230109205336.3665937-1-surenb@google.com/ (local)
RFC: https://lore.kernel.org/all/20220901173516.702122-1-surenb@google.com/ (local)

LWN article describing the feature:
https://lwn.net/Articles/906852/

Per-vma locks idea that was discussed during SPF [1] discussion at LSF/MM
last year [2], which concluded with suggestion that “a reader/writer
semaphore could be put into the VMA itself; that would have the effect of
using the VMA as a sort of range lock. There would still be contention at
the VMA level, but it would be an improvement.” This patchset implements
this suggested approach.

I took the patches for a spin on a 2-socket 32 core (64 threads) system
with Intel 8336C (Ice Lake) and 512GB of RAM.

For the initial testing, "pft-threads" from the mm-tests suite[0] was
used. The test mmaps a memory region (~100GB on the test system) and
triggers access by a number of threads executing in parallel. For each
degree of parallelism, the test is repeated 10 times to get a better
feel for the behaviour. Below is an excerpt of the harmonic mean
reported by 'compare_kernel' script[1] included with mm-tests.

The first column is results for mm-unstable as of 2023-02-10, the second
column is the patches posted here while the third column includes
optimizations to reclaim some of the observed regression.

From the results, there is a drop in page fault/second for low number of
CPUs but good improvement with higher CPUs.

                                        6.2.0-rc4                6.2.0-rc4                6.2.0-rc4
                             mm-unstable-20230210                   pvl-v2               pvl-v2+opt

Hmean     faults/cpu-1     898792.9338 (   0.00%)   894597.0474 *  -0.47%*   895933.2782 *  -0.32%*
Hmean     faults/cpu-4     751903.9803 (   0.00%)   677764.2975 *  -9.86%*   688643.8163 *  -8.41%*
Hmean     faults/cpu-7     612275.5663 (   0.00%)   565363.4137 *  -7.66%*   597538.9396 *  -2.41%*
Hmean     faults/cpu-12    434460.9074 (   0.00%)   410974.2708 *  -5.41%*   452501.4290 *   4.15%*
Hmean     faults/cpu-21    291475.5165 (   0.00%)   293936.8460 (   0.84%)   308712.2434 *   5.91%*
Hmean     faults/cpu-30    218021.3980 (   0.00%)   228265.0559 *   4.70%*   241897.5225 *  10.95%*
Hmean     faults/cpu-48    141798.5030 (   0.00%)   162322.5972 *  14.47%*   166081.9459 *  17.13%*
Hmean     faults/cpu-79     90060.9577 (   0.00%)   107028.7779 *  18.84%*   109810.4488 *  21.93%*
Hmean     faults/cpu-110    64729.3561 (   0.00%)    80597.7246 *  24.51%*    83134.0679 *  28.43%*
Hmean     faults/cpu-128    55740.1334 (   0.00%)    68395.4426 *  22.70%*    69248.2836 *  24.23%*

Hmean     faults/sec-1     898781.7694 (   0.00%)   894247.3174 *  -0.50%*   894440.3118 *  -0.48%*
Hmean     faults/sec-4    2965588.9697 (   0.00%)  2683651.5664 *  -9.51%*  2726450.9710 *  -8.06%*
Hmean     faults/sec-7    4144512.3996 (   0.00%)  3891644.2128 *  -6.10%*  4099918.8601 (  -1.08%)
Hmean     faults/sec-12   4969513.6934 (   0.00%)  4829731.4355 *  -2.81%*  5264682.7371 *   5.94%*
Hmean     faults/sec-21   5814379.4789 (   0.00%)  5941405.3116 *   2.18%*  6263716.3903 *   7.73%*
Hmean     faults/sec-30   6153685.3709 (   0.00%)  6489311.6634 *   5.45%*  6910843.5858 *  12.30%*
Hmean     faults/sec-48   6197953.1327 (   0.00%)  7216320.7727 *  16.43%*  7412782.2927 *  19.60%*
Hmean     faults/sec-79   6167135.3738 (   0.00%)  7425927.1022 *  20.41%*  7637042.2198 *  23.83%*
Hmean     faults/sec-110  6264768.2247 (   0.00%)  7813329.3863 *  24.72%*  7984344.4005 *  27.45%*
Hmean     faults/sec-128  6460727.8216 (   0.00%)  7875664.8999 *  21.90%*  8049910.3601 *  24.60%*

The above workload represent the worst case with regards to per-VMA
locks as it creates a single large VMA. As a follow-up, I modified
pft[2] to create a VMA per thread to understand the behaviour in
scenarios where per-VMA locks should show the most benefit.

                                        6.2.0-rc4                6.2.0-rc4                6.2.0-rc4
                             mm-unstable-20230210                   pvl-v2               pvl-v2+opt

Hmean     faults/cpu-1     905497.4354 (   0.00%)   888736.5570 *  -1.85%*   888695.2675 *  -1.86%*
Hmean     faults/cpu-4     758519.2719 (   0.00%)   812103.1991 *   7.06%*   825077.9277 *   8.77%*
Hmean     faults/cpu-7     617153.8038 (   0.00%)   729943.4518 *  18.28%*   770872.3161 *  24.91%*
Hmean     faults/cpu-12    424848.5266 (   0.00%)   550357.2856 *  29.54%*   597478.5634 *  40.63%*
Hmean     faults/cpu-21    290142.9988 (   0.00%)   383668.3190 *  32.23%*   433376.8959 *  49.37%*
Hmean     faults/cpu-30    218705.2915 (   0.00%)   299888.5533 *  37.12%*   342640.6153 *  56.67%*
Hmean     faults/cpu-48    142842.3372 (   0.00%)   206498.2605 *  44.56%*   240306.3442 *  68.23%*
Hmean     faults/cpu-79     90706.1425 (   0.00%)   160006.6800 *  76.40%*   185298.4326 * 104.28%*
Hmean     faults/cpu-110    67011.9297 (   0.00%)   143536.0062 * 114.19%*   162688.8015 * 142.78%*
Hmean     faults/cpu-128    55986.4986 (   0.00%)   136550.8760 * 143.90%*   152718.8713 * 172.78%*

Hmean     faults/sec-1     905492.1265 (   0.00%)   887244.6592 *  -2.02%*   887775.6079 *  -1.96%*
Hmean     faults/sec-4    2994284.4204 (   0.00%)  3154236.9408 *   5.34%*  3221994.8465 *   7.60%*
Hmean     faults/sec-7    4177411.3461 (   0.00%)  4933286.4045 *  18.09%*  5202347.2077 *  24.54%*
Hmean     faults/sec-12   4892848.3633 (   0.00%)  6054577.0988 *  23.74%*  6511987.1142 *  33.09%*
Hmean     faults/sec-21   5823534.1820 (   0.00%)  7637637.4162 *  31.15%*  8553362.3513 *  46.88%*
Hmean     faults/sec-30   6247210.8414 (   0.00%)  8598150.6717 *  37.63%*  9799696.0945 *  56.87%*
Hmean     faults/sec-48   6274617.1419 (   0.00%)  9467132.3699 *  50.88%* 11049401.9072 *  76.10%*
Hmean     faults/sec-79   6187291.4971 (   0.00%) 11919062.5284 *  92.64%* 13420825.3820 * 116.91%*
Hmean     faults/sec-110  6454542.3239 (   0.00%) 15050228.1869 * 133.17%* 16667873.7618 * 158.23%*
Hmean     faults/sec-128  6472970.8548 (   0.00%) 16647275.6575 * 157.18%* 18680029.3714 * 188.59%*

As expected, the tests highlight the improved scalability as core count
increases.

[0] https://github.com/gormanm/mmtests
[1] https://github.com/gormanm/mmtests/blob/master/compare-kernels.sh

[2] https://github.com/gormanm/pft/pull/1/commits/8fe554a3d8b4f5947cd00d4b46f97178b8ba8752

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