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This is identical to CONFIG_PREEMPT, and so should result in similar performance. Comparing scheduling/IPC workload: # perf stat -a -e cs --repeat 10 -- perf bench sched messaging -g 20 -t -l 5000 PREEMPT_AUTO, preempt=full 3,080,508 context-switches ( +- 0.64% ) 3.65171 +- 0.00654 seconds time elapsed ( +- 0.18% ) PREEMPT_DYNAMIC, preempt=full 3,087,527 context-switches ( +- 0.33% ) 3.60163 +- 0.00633 seconds time elapsed ( +- 0.18% ) Looking at the breakup between voluntary and involuntary context-switches, we see almost identical behaviour as well. PREEMPT_AUTO, preempt=full 2087910.00 +- 34720.95 voluntary context-switches ( +- 1.660% ) 784437.60 +- 19827.79 involuntary context-switches ( +- 2.520% ) PREEMPT_DYNAMIC, preempt=full 2102879.60 +- 22767.11 voluntary context-switches ( +- 1.080% ) 801189.90 +- 21324.18 involuntary context-switches ( +- 2.660% ) Comparing kernbench half load (-j 32), we see that both voluntary and involuntary context switches, and their stdev is fairly similar. So is the percentage of CPU taken and various process times. # Half load (-j 32) PREEMPT_AUTO PREEMPT_DYNAMIC wall 74.45 +- 0.39 sec ( +- 0.53% ) 74.08 +- 0.20 sec ( +- 0.27% ) utime 1419.68 +- 5.12 sec ( +- 0.36% ) 1419.76 +- 3.63 sec ( +- 0.25% ) stime 250.56 +- 1.08 sec ( +- 0.43% ) 248.94 +- 0.80 sec ( +- 0.32% ) %cpu 2243.20 +- 19.57 ( +- 0.87% ) 2251.80 +- 11.12 ( +- 0.49% ) inv-csw 20286.60 +- 547.48 ( +- 2.69% ) 20175.60 +- 214.20 ( +- 1.06% ) vol-csw 187688.00 +- 5097.26 ( +- 2.71% ) 182914.00 +- 2525.59 ( +- 1.38% ) Same for kernbench optimal and maximal loads. # Optimal load (-j 256) PREEMPT_AUTO PREEMPT_DYNAMIC wall 65.10 +- 0.09 sec ( +- 0.14% ) 65.11 +- 0.27 sec ( +- 0.42% ) utime 1875.03 +- 479.98 sec ( +- 25.59% ) 1874.55 +- 479.39 sec ( +- 25.57% ) stime 297.70 +- 49.68 sec ( +- 16.69% ) 297.04 +- 50.69 sec ( +- 17.06% ) %cpu 3175.60 +- 982.93 ( +- 30.95% ) 3179.40 +- 977.87 ( +- 30.75% ) inv-csw 391147.00 +- 390941.00 ( +- 99.94% ) 392298.00 +- 392268.00 ( +- 99.99% ) vol-csw 212039.00 +- 26419.90 ( +- 12.45% ) 211349.00 +- 30227.30 ( +- 14.30% ) # Maximal load (-j 256) PREEMPT_AUTO PREEMPT_DYNAMIC wall 66.55 +- 0.34 sec ( +- 0.51% ) 66.41 +- 0.72 sec ( +- 1.09% ) utime 2028.83 +- 445.86 sec ( +- 21.97% ) 2027.59 +- 444.89 sec ( +- 21.94% ) stime 316.16 +- 48.29 sec ( +- 15.27% ) 313.97 +- 47.61 sec ( +- 15.16% ) %cpu 3463.93 +- 894.12 ( +- 25.81% ) 3465.33 +- 889.04 ( +- 25.65% ) inv-csw 491115.00 +- 345936.00 ( +- 70.43% ) 492028.00 +- 346745.00 ( +- 70.47% ) vol-csw 200509.00 +- 32922.60 ( +- 16.41% ) 187447.00 +- 42567.20 ( +- 22.70% ) Cc: Ingo Molnar Cc: Peter Ziljstra Cc: Juri Lelli Cc: Vincent Guittot Originally-by: Thomas Gleixner Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/ Signed-off-by: Ankur Arora --- kernel/sched/core.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2d33f3ff51a3..aaa87d5fecdd 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1035,8 +1035,9 @@ void wake_up_q(struct wake_q_head *head) * For preemption models other than PREEMPT_AUTO: always schedule * eagerly. * - * For PREEMPT_AUTO: schedule idle threads eagerly, allow everything - * else, whether running in user or kernel context, to finish its time + * For PREEMPT_AUTO: schedule idle threads eagerly, and under full + * preemption all tasks eagerly. Otherwise, allow everything else, + * whether running in user or kernel context, to finish its time * quanta, and mark for rescheduling at the next exit to user. * * Note: to avoid the hog problem, where the user does not relinquish @@ -1052,6 +1053,9 @@ static resched_t resched_opt_translate(struct task_struct *curr, if (opt == RESCHED_FORCE) return NR_now; + if (preempt_model_preemptible()) + return NR_now; + if (is_idle_task(curr)) return NR_now; @@ -8982,7 +8986,9 @@ static void __sched_dynamic_update(int mode) pr_warn("%s: preempt=full is not recommended with CONFIG_PREEMPT_RCU=n", PREEMPT_MODE); - preempt_dynamic_mode = preempt_dynamic_undefined; + if (mode != preempt_dynamic_mode) + pr_info("%s: full\n", PREEMPT_MODE); + preempt_dynamic_mode = mode; break; } }