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Commit:d670ec1317("posix-cpu-timers: Cure SMP wobbles") started accounting thread group tasks pending runtime in thread_group_cputime(). Another commit:6e998916df("sched/cputime: Fix clock_nanosleep()/clock_gettime() inconsistency") updated scheduler runtime statistics (call update_curr()) when reading task pending runtime. Those changes cause bad performance of SYS_times() and SYS_clock_gettimes(CLOCK_PROCESS_CPUTIME_ID) syscalls, especially on larger systems with many CPUs. While we would like to have cpuclock monotonicity kept i.e. have problems fixed by above commits stay fixed, we also would like to have good performance. However when we notice that change from commitd670ec1317is not longer needed to solve problem addressed by that commit, because of change from the second commit6e998916df, we can get room for optimization. Since we update task while reading it's pending runtime in task_sched_runtime(), clock_gettime(CLOCK_PROCESS_CPUTIME_ID) will see updated values and on testcase fromd670ec1317process cpuclock will not be smaller than thread cpuclock. I tested the patch on testcases from commitsd670ec1317,6e998916dfand some other cpuclock/cputimers testcases and did not found cpuclock monotonicity problems or other malfunction. This patch has the drawback that we will not provide thread group cputime up-to-date to the last moment. For example when arming cputime timer, we will arm it with possibly a bit outdated values and that timer will trigger earlier compared to behaviour without the patch. However that was the behaviour befored670ec1317commit (kernel v3.1) so it's unlikely to affect applications. Patch improves related syscall performance, as measured by Giovanni's benchmarks described in commit:6075620b05("sched/cputime: Mitigate performance regression in times()/clock_gettime()") The benchmark results are: SYS_clock_gettime(): threads 4.7-rc7 3.18-rc3 4.7-rc7 + prefetch 4.7-rc7 + patch (pre-6e998916dfe3) 2 3.48 2.23 ( 35.68%) 3.06 ( 11.83%) 1.08 ( 68.81%) 5 3.33 2.83 ( 14.84%) 3.25 ( 2.40%) 0.71 ( 78.55%) 8 3.37 2.84 ( 15.80%) 3.26 ( 3.30%) 0.56 ( 83.49%) 12 3.32 3.09 ( 6.69%) 3.37 ( -1.60%) 0.42 ( 87.28%) 21 4.01 3.14 ( 21.70%) 3.90 ( 2.74%) 0.35 ( 91.35%) 30 3.63 3.28 ( 9.75%) 3.36 ( 7.41%) 0.28 ( 92.23%) 48 3.71 3.02 ( 18.69%) 3.11 ( 16.27%) 0.39 ( 89.39%) 79 3.75 2.88 ( 23.23%) 3.16 ( 15.74%) 0.46 ( 87.76%) 110 3.81 2.95 ( 22.62%) 3.25 ( 14.80%) 0.56 ( 85.41%) 128 3.88 3.05 ( 21.28%) 3.31 ( 14.76%) 0.62 ( 84.10%) SYS_times(): threads 4.7-rc7 3.18-rc3 4.7-rc7 + prefetch 4.7-rc7 + patch (pre-6e998916dfe3) 2 3.65 2.27 ( 37.94%) 3.25 ( 11.03%) 1.62 ( 55.71%) 5 3.45 2.78 ( 19.34%) 3.17 ( 7.92%) 2.33 ( 32.28%) 8 3.52 2.79 ( 20.66%) 3.22 ( 8.69%) 2.06 ( 41.44%) 12 3.29 3.02 ( 8.33%) 3.36 ( -2.04%) 2.00 ( 39.18%) 21 4.07 3.10 ( 23.86%) 3.92 ( 3.78%) 2.07 ( 49.18%) 30 3.87 3.33 ( 13.80%) 3.40 ( 12.17%) 1.89 ( 51.12%) 48 3.79 2.96 ( 21.94%) 3.16 ( 16.61%) 1.69 ( 55.46%) 79 3.88 2.88 ( 25.82%) 3.28 ( 15.42%) 1.60 ( 58.81%) 110 3.90 2.98 ( 23.73%) 3.38 ( 13.35%) 1.73 ( 55.61%) 128 4.00 3.10 ( 22.40%) 3.38 ( 15.45%) 1.66 ( 58.52%) Reported-and-tested-by: Giovanni Gherdovich <ggherdovich@suse.cz> Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Mike Galbraith <mgalbraith@suse.de> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Rik van Riel <riel@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Wanpeng Li <wanpeng.li@hotmail.com> Link: http://lkml.kernel.org/r/20160817093043.GA25206@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>