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Date: Mon, 12 Feb 2024 21:55:49 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, peterz@...radead.org, torvalds@...ux-foundation.org,
paulmck@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
willy@...radead.org, mgorman@...e.de, jpoimboe@...nel.org,
mark.rutland@....com, jgross@...e.com, andrew.cooper3@...rix.com,
bristot@...nel.org, mathieu.desnoyers@...icios.com,
geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
anton.ivanov@...bridgegreys.com, mattst88@...il.com,
krypton@...ich-teichert.org, rostedt@...dmis.org,
David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
Ankur Arora <ankur.a.arora@...cle.com>
Subject: [PATCH 25/30] sched: support preempt=full under PREEMPT_AUTO
The default preemption policy for preempt-full under PREEMPT_AUTO is
to minimize latency, and thus to always schedule eagerly. 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 <mingo@...hat.com>
Cc: Peter Ziljstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Originally-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
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;
}
}
--
2.31.1
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