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[RFC/RFT V2 PATCH 0/5] Improve scheduler scalability for fast path




Current select_idle_sibling first tries to find a fully idle core using
select_idle_core which can potentially search all cores and if it fails it
finds any idle cpu using select_idle_cpu. select_idle_cpu can potentially
search all cpus in the llc domain. This doesn't scale for large llc domains
and will only get worse with more cores in future.

This patch solves the scalability problem by:
 - Setting an upper and lower limit of idle cpu search in select_idle_cpu
   to keep search time low and constant
 - Adding a new sched feature SIS_CORE to disable select_idle_core

Additionally it also introduces a new per-cpu variable next_cpu to track
the limit of search so that every time search starts from where it ended.
This rotating search window over cpus in LLC domain ensures that idle
cpus are eventually found in case of high load.

Following are the performance numbers with various benchmarks with SIS_CORE
true (idle core search enabled).

Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine
(lower is better):
groups  baseline           %stdev  patch           %stdev
1       0.5816             8.94    0.5903 (-1.5%)  11.28 
2       0.6428             10.64   0.5843 (9.1%)   4.93
4       1.0152             1.99    0.9965 (1.84%)  1.83
8       1.8128             1.4     1.7921 (1.14%)  1.76
16      3.1666             0.8     3.1345 (1.01%)  0.81
32      5.6084             0.83    5.5677 (0.73%)  0.8 

Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
threads baseline     %stdev     patch	           %stdev
8       2095.45      1.82 	2102.6 (0.34%)	   2.11
16      4218.45      0.06       4221.35 (0.07%)    0.38
32      7531.36      0.49       7607.18 (1.01%)    0.25
48      10206.42     0.21       10324.26 (1.15%)   0.13
64      12053.73     0.1        12158.3 (0.87%)    0.24
128     14810.33     0.04       14840.4 (0.2%)	   0.38

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine
(normalized, higher is better):
users   baseline        %stdev  patch            %stdev
20      1               0.9     1.0068 (0.68%)   0.27
40      1               0.8     1.0103 (1.03%)   1.24
60      1               0.34    1.0178 (1.78%)   0.49
80      1               0.53    1.0092 (0.92%)   1.5
100     1               0.79    1.0090 (0.9%)    0.88
120     1               0.06    1.0048 (0.48%)   0.72
140     1               0.22    1.0116 (1.16%)   0.05
160     1               0.57    1.0264 (2.64%)   0.67
180     1               0.81    1.0194 (1.94%)   0.91
200     1               0.44    1.028 (2.8%)     3.09
220     1               1.74    1.0229 (2.29%)   0.21

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with
message size = 8k (higher is better):
threads baseline        %stdev  patch            %stdev
8       45.36           0.43    46.28 (2.01%)    0.29
16      87.81           0.82    89.67 (2.12%)    0.38
32      151.19          0.02    153.5 (1.53%)    0.41
48      190.2           0.21    194.79 (2.41%)   0.07
64      190.42          0.35    202.9 (6.55%)    1.66
128     323.86          0.28    343.56 (6.08%)   1.34 

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       629.8           603.83 (-4.12%)
2       1159.65         1155.75 (-0.34%)
4       2121.61         2093.99 (-1.3%)
8       2620.52         2641.51 (0.8%)
16      2879.31         2897.6 (0.64%)
32      2791.24         2936.47 (5.2%)
64      1853.07         1894.74 (2.25%)
128     1484.95         1494.29 (0.63%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine
(higher is better):
clients baseline        patch
1       256.41          255.8 (-0.24%)
2       509.89          504.52 (-1.05%)
4       999.44          1003.74 (0.43%)
8       1982.7          1976.42 (-0.32%)
16      3891.51         3916.04 (0.63%)
32      6819.24         6845.06 (0.38%)
64      8542.95         8568.28 (0.3%)
128     15277.6         15754.6 (3.12%)

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44
tasks (lower is better):
percentile      baseline      %stdev   patch             %stdev
50              94            2.82     92 (2.13%)        2.17
75              124           2.13     122 (1.61%)       1.42
90              152           1.74     151 (0.66%)       0.66 
95              171           2.11     170 (0.58%)       0
99              512.67        104.96   208.33 (59.36%)   1.2
99.5            2296          82.55    3674.66 (-60.05%) 22.19
99.9            12517.33      2.38     12784 (-2.13%)    0.66

Hackbench process on 2 socket, 16 core and 128 threads SPARC machine
(lower is better):  
groups  baseline           %stdev  patch             %stdev 
1       1.3085             6.65    1.2213 (6.66%)    10.32 
2       1.4559             8.55    1.5048 (-3.36%)   4.72
4       2.6271             1.74    2.5532 (2.81%)    2.02
8       4.7089             3.01    4.5118 (4.19%)    2.74
16      8.7406             2.25    8.6801 (0.69%)    4.78
32      17.7835            1.01    16.759 (5.76%)    1.38
64	36.1901		   0.65	   34.6652 (4.21%)   1.24
128	72.6585		   0.51	   70.9762 (2.32%)   0.9

Following are the performance numbers with various benchmarks with SIS_CORE
false (idle core search disabled). This improves throughput of certain
workloads but increases latency of other workloads.
                                                                           
Hackbench process on 2 socket, 44 core and 88 threads Intel x86 machine 
(lower is better):
groups  baseline           %stdev  patch            %stdev
1       0.5816             8.94    0.5835 (-0.33%)  8.21  
2       0.6428             10.64   0.5752 (10.52%)  4.05  
4       1.0152             1.99    0.9946 (2.03%)   2.56  
8       1.8128             1.4     1.7619 (2.81%)   1.88  
16      3.1666             0.8     3.1275 (1.23%)   0.42  
32      5.6084             0.83    5.5856 (0.41%)   0.89   
 
Sysbench MySQL on 2 socket, 44 core and 88 threads Intel x86 machine 
(higher is better):
threads baseline     %stdev     patch              %stdev 
8       2095.45      1.82       2084.72 (-0.51%)   1.65
16      4218.45      0.06       4179.69 (-0.92%)   0.18
32      7531.36      0.49       7623.18 (1.22%)    0.39
48      10206.42     0.21       10159.16 (-0.46%)  0.21
64      12053.73     0.1        12087.21 (0.28%)   0.19
128     14810.33     0.04       14894.08 (0.57%)   0.08

Oracle DB on 2 socket, 44 core and 88 threads Intel x86 machine            
(normalized, higher is better):                                            
users   baseline        %stdev  patch            %stdev                    
20      1               0.9     1.0056 (0.56%)   0.34
40      1               0.8     1.0173 (1.73%)   0.13
60      1               0.34    0.9995 (-0.05%)  0.85
80      1               0.53    1.0175 (1.75%)   1.56
100     1               0.79    1.0151 (1.51%)   1.31
120     1               0.06    1.0244 (2.44%)   0.5 
140     1               0.22    1.034 (3.4%)     0.66
160     1               0.57    1.0362 (3.62%)   0.07
180     1               0.81    1.041 (4.1%)     0.8
200     1               0.44    1.0233 (2.33%)   1.4  
220     1               1.74    1.0125 (1.25%)   1.41

Uperf pingpong on 2 socket, 44 core and 88 threads Intel x86 machine with  
message size = 8k (higher is better):
threads baseline        %stdev  patch            %stdev 
8       45.36           0.43    46.94 (3.48%)    0.2  
16      87.81           0.82    91.75 (4.49%)    0.43 
32      151.19          0.02    167.74 (10.95%)  1.29 
48      190.2           0.21    200.57 (5.45%)   0.89 
64      190.42          0.35    226.74 (19.07%)  1.79 
128     323.86          0.28    348.12 (7.49%)   0.77 

Dbench on 2 socket, 44 core and 88 threads Intel x86 machine               
(higher is better):         
clients baseline        patch 
1       629.8           600.19 (-4.7%)  
2       1159.65         1162.07 (0.21%) 
4       2121.61         2112.27 (-0.44%)
8       2620.52         2645.55 (0.96%) 
16      2879.31         2828.87 (-1.75%)
32      2791.24         2760.97 (-1.08%)
64      1853.07         1747.66 (-5.69%)
128     1484.95         1459.81 (-1.69%)

Tbench on 2 socket, 44 core and 88 threads Intel x86 machine               
(higher is better):
clients baseline        patch
1       256.41          258.11 (0.67%)  
2       509.89          509.13 (-0.15%) 
4       999.44          1016.58 (1.72%) 
8       1982.7          2006.53 (1.2%)  
16      3891.51         3964.43 (1.87%) 
32      6819.24         7376.92 (8.18%) 
64      8542.95         9660.45 (13.08%) 
128     15277.6         15438.4 (1.05%) 

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44     
tasks (lower is better):
percentile      baseline      %stdev   patch                %stdev
50              94            2.82     94.67 (-0.71%)       2.2  
75              124           2.13     124.67 (-0.54%)      1.67 
90              152           1.74     154.33 (-1.54%)      0.75 
95              171           2.11     176.67 (-3.31%)      0.86 
99              512.67        104.96   4130.33 (-705.65%)   79.41
99.5            2296          82.55    10066.67 (-338.44%)  26.15
99.9            12517.33      2.38     12869.33 (-2.81%)    0.8 
 
Hackbench process on 2 socket, 16 core and 128 threads SPARC machine       
(lower is better): 
groups  baseline           %stdev  patch             %stdev
1       1.3085             6.65    1.2514 (4.36%)    11.1
2       1.4559             8.55    1.5433 (-6%)      3.05
4       2.6271             1.74    2.5626 (2.5%)     2.69
8       4.7089             3.01    4.5316 (3.77%)    2.95
16      8.7406             2.25    8.6585 (0.94%)    2.91
32      17.7835            1.01    17.175 (3.42%)    1.38
64      36.1901            0.65    35.5294 (1.83%)   1.02
128     72.6585            0.51    71.8821 (1.07%)   1.05

Following are the schbench performance numbers with SIS_CORE false and
SIS_PROP false. This recovers the latency increase by having SIS_CORE
false.

Schbench on 2 socket, 44 core and 88 threads Intel x86 machine with 44     
tasks (lower is better):
percentile      baseline      %stdev   patch               %stdev
50              94            2.82     93.33 (0.71%)       1.24  
75              124           2.13     122.67 (1.08%)      1.7   
90              152           1.74     149.33 (1.75%)      2.35  
95              171           2.11     167 (2.34%)         2.74
99              512.67        104.96   206 (59.82%)        8.86
99.5            2296          82.55    3121.67 (-35.96%)   97.37 
99.9            12517.33      2.38     12592 (-0.6%)       1.67

Changes since v1
 - Compute the upper and lower limit based on number of cpus in a core
 - Split up the search limit and search window rotation into separate
   patches
 - Add new sched feature to have option of disabling idle core search

subhra mazumdar (5):
  sched: limit cpu search in select_idle_cpu
  sched: introduce per-cpu var next_cpu to track search limit
  sched: rotate the cpu search window for better spread
  sched: add sched feature to disable idle core search
  sched: SIS_CORE to disable idle core search

 kernel/sched/core.c     |  2 ++
 kernel/sched/fair.c     | 31 +++++++++++++++++++++++--------
 kernel/sched/features.h |  1 +
 kernel/sched/sched.h    |  1 +
 4 files changed, 27 insertions(+), 8 deletions(-)

-- 
2.9.3