On Mon, Nov 21, 2022 at 11:51:40AM +0800, Zhouyi Zhou wrote:
During CPU-hotplug torture (CONFIG_NO_HZ_FULL=y), if we try to
offline tick_do_timer_cpu, the operation will fail because in
function tick_nohz_cpu_down:
if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
return -EBUSY;
Above bug was first discovered in torture tests performed in PPC VM
of Open Source Lab of Oregon State University, and reproducable in RISC-V
and X86-64 (with additional kernel commandline cpu0_hotplug).
In this patch, we avoid offline tick_do_timer_cpu by distribute
the offlining cpu among remaining cpus.
Signed-off-by: Zhouyi Zhou <redacted>
Good show chasing this down!
A couple of questions below.
quoted hunk ↗ jump to hunk
---
include/linux/tick.h | 1 +
kernel/time/tick-common.c | 1 +
kernel/time/tick-internal.h | 1 -
kernel/torture.c | 10 ++++++++++
4 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/include/linux/tick.h b/include/linux/tick.h
index bfd571f18cfd..23cc0b205853 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -14,6 +14,7 @@
#include <linux/rcupdate.h>
#ifdef CONFIG_GENERIC_CLOCKEVENTS
+extern int tick_do_timer_cpu __read_mostly;
extern void __init tick_init(void);
/* Should be core only, but ARM BL switcher requires it */
extern void tick_suspend_local(void);
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 46789356f856..87b9b9afa320 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -48,6 +48,7 @@ ktime_t tick_next_period;
* procedure also covers cpu hotplug.
*/
int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
+EXPORT_SYMBOL_GPL(tick_do_timer_cpu);
#ifdef CONFIG_NO_HZ_FULL
/*
* tick_do_timer_boot_cpu indicates the boot CPU temporarily owns
diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 649f2b48e8f0..8953dca10fdd 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -15,7 +15,6 @@
DECLARE_PER_CPU(struct tick_device, tick_cpu_device);
extern ktime_t tick_next_period;
-extern int tick_do_timer_cpu __read_mostly;
extern void tick_setup_periodic(struct clock_event_device *dev, int broadcast);
extern void tick_handle_periodic(struct clock_event_device *dev);
diff --git a/kernel/torture.c b/kernel/torture.c
index 789aeb0e1159..bccbdd33dda2 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -33,6 +33,7 @@
#include <linux/delay.h>
#include <linux/stat.h>
#include <linux/slab.h>
+#include <linux/tick.h>
#include <linux/trace_clock.h>
#include <linux/ktime.h>
#include <asm/byteorder.h>
@@ -358,7 +359,16 @@ torture_onoff(void *arg)
schedule_timeout_interruptible(HZ / 10);
continue;
}
+#ifdef CONFIG_NO_HZ_FULL
+ /* do not offline tick do timer cpu */
+ if (tick_nohz_full_running) {
+ cpu = (torture_random(&rand) >> 4) % maxcpu;
+ if (cpu >= tick_do_timer_cpu)
Why is this ">=" instead of "=="?
quoted hunk ↗ jump to hunk
+ cpu = (cpu + 1) % (maxcpu + 1);
+ } else
+#else
cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
+#endif
What happens if the value of tick_do_timer_cpu changes between the time of
the check above and the call to torture_offline() below? Alternatively,
how is such a change in value prevented?
Thanx, Paul
if (!torture_offline(cpu,
&n_offline_attempts, &n_offline_successes,
&sum_offline, &min_offline, &max_offline))
--
2.34.1