[RFC] sched/fair: hard lockup in sched_cfs_period_timer

[Phil Auld <pauld@redhat.com>]

Hi,

I have a reproducible case of this: 

[  217.264946] NMI watchdog: Watchdog detected hard LOCKUP on cpu 24
[  217.264948] Modules linked in: sunrpc iTCO_wdt gpio_ich iTCO_vendor_support intel_powerclamp coretemp kvm_intel kvm ipmi_ssif irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel ipmi_si intel_cstate joydev ipmi_devintf pcspkr hpilo intel_uncore sg hpwdt ipmi_msghandler acpi_power_meter i7core_edac lpc_ich acpi_cpufreq ip_tables xfs libcrc32c sr_mod sd_mod cdrom ata_generic radeon i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm ata_piix drm serio_raw crc32c_intel hpsa myri10ge libata dca scsi_transport_sas netxen_nic dm_mirror dm_region_hash dm_log dm_mod
[  217.264964] CPU: 24 PID: 46684 Comm: myapp Not tainted 5.0.0-rc7+ #25
[  217.264965] Hardware name: HP ProLiant DL580 G7, BIOS P65 08/16/2015
[  217.264965] RIP: 0010:tg_nop+0x0/0x10
[  217.264966] Code: 83 c9 08 f0 48 0f b1 0f 48 39 c2 74 0e 48 89 c2 f7 c2 00 00 20 00 75 dc 31 c0 c3 b8 01 00 00 00 c3 66 0f 1f 84 00 00 00 00 00 <66> 66 66 66 90 31 c0 c3 0f 1f 84 00 00 00 00 00 66 66 66 66 90 8b
[  217.264967] RSP: 0000:ffff976a7f703e48 EFLAGS: 00000087
[  217.264967] RAX: ffff976a7c7c8f00 RBX: ffff976a6f4fad00 RCX: ffff976a7c7c90f0
[  217.264968] RDX: ffff976a6f4faee0 RSI: ffff976a7f961f00 RDI: ffff976a6f4fad00
[  217.264968] RBP: ffff976a7f961f00 R08: 0000000000000002 R09: ffffffad2c9b3331
[  217.264969] R10: 0000000000000000 R11: 0000000000000000 R12: ffff976a7c7c8f00
[  217.264969] R13: ffffffffb2305c00 R14: 0000000000000000 R15: ffffffffb22f8510
[  217.264970] FS:  00007f6240678740(0000) GS:ffff976a7f700000(0000) knlGS:0000000000000000
[  217.264970] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  217.264971] CR2: 00000000006dee20 CR3: 000000bf2bffc005 CR4: 00000000000206e0
[  217.264971] Call Trace:
[  217.264971]  <IRQ>
[  217.264972]  walk_tg_tree_from+0x29/0xb0
[  217.264972]  unthrottle_cfs_rq+0xe0/0x1a0
[  217.264972]  distribute_cfs_runtime+0xd3/0xf0
[  217.264973]  sched_cfs_period_timer+0xcb/0x160
[  217.264973]  ? sched_cfs_slack_timer+0xd0/0xd0
[  217.264973]  __hrtimer_run_queues+0xfb/0x270
[  217.264974]  hrtimer_interrupt+0x122/0x270
[  217.264974]  smp_apic_timer_interrupt+0x6a/0x140
[  217.264975]  apic_timer_interrupt+0xf/0x20
[  217.264975]  </IRQ>
[  217.264975] RIP: 0033:0x7f6240125fe5
[  217.264976] Code: 44 17 d0 f3 44 0f 7f 47 30 f3 44 0f 7f 44 17 c0 48 01 fa 48 83 e2 c0 48 39 d1 74 a3 66 0f 1f 84 00 00 00 00 00 66 44 0f 7f 01 <66> 44 0f 7f 41 10 66 44 0f 7f 41 20 66 44 0f 7f 41 30 48 83 c1 40
...

There's more but this is the juicy part. 

The reproducer involves a large number of cgroups in a hierarchy and a very low cfs_period_us 
setting. 

The guts of sched_cfs_period_timer is this forever loop:

	raw_spin_lock(&cfs_b->lock);
	for (;;) {
		overrun = hrtimer_forward_now(timer, cfs_b->period);
		if (!overrun)
			break;

		idle = do_sched_cfs_period_timer(cfs_b, overrun);
	}
	if (idle)
		cfs_b->period_active = 0;
	raw_spin_unlock(&cfs_b->lock);


There's no guarantee that hrtimer_forward_now() will ever return 0 and also 
do_sched_cfs_period_timer() will drop and re-acquire the cfs_b->lock to call 
distribute_cfs_runtime. 

I considered code to tune the period and quota up (proportionally so they have the same 
relative effect) but I did not like that because of the above statement and the fact that 
it would be changing the valid values the user configured. I have two versions that do that 
differently but they both still call out for protection from the forever loop. I think they 
add complexity for no real gain.

For my current testing I'm using a more direct but less elegant approach of simply limiting 
the number of times the loop can execute. This has the potential to skip handling a period 
I think but will prevent the problem reliably. I'm not sure how many iterations this loop 
was expected to take. I'm seeing numbers in the low thousands at times.

A more complete fix to make sure do_sched_cfs_period_timer never takes longer than period 
would be good but I'm not sure what that would be and we still have this potential forever
loop. 

Below is the bandaid version. 
  
Thoughts? 


Cheers,
Phil

---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 310d0637fe4b..33e55620f891 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4859,12 +4859,16 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
        return HRTIMER_NORESTART;
 }
 
+/* This is pretty arbitrary just to escape the "forever" loop before the watchdog
+ * kills us as there is no guarantee hrtimer_forward_now ever returns 0. */
+#define CFS_PERIOD_TIMER_EXIT_COUNT 100
 static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 {
        struct cfs_bandwidth *cfs_b =
                container_of(timer, struct cfs_bandwidth, period_timer);
        int overrun;
        int idle = 0;
+       int count = 0;
 
        raw_spin_lock(&cfs_b->lock);
        for (;;) {
@@ -4872,6 +4876,14 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
                if (!overrun)
                        break;
 
+               if (++count > CFS_PERIOD_TIMER_EXIT_COUNT) {
+                       pr_warn_ratelimited("cfs_period_timer(%d): too many overruns. Consider raising cfs_period_us (%lld)\n",
+                               smp_processor_id(), cfs_b->period/NSEC_PER_USEC);
+                       // Make sure we restart the timer.
+                       idle = 0;
+                       break;
+               }
+
                idle = do_sched_cfs_period_timer(cfs_b, overrun);
        }
        if (idle)


--