On Wed, May 18, 2022 at 01:51:52PM +0100, Mel Gorman wrote:
> On Tue, May 17, 2022 at 07:35:07PM -0400, Qian Cai wrote:
> > On Thu, May 12, 2022 at 09:50:37AM +0100, Mel Gorman wrote:
> > > Changelog since v2
> > > o More conversions from page->lru to page->[pcp_list|buddy_list]
> > > o Additional test results in changelogs
> > >
> > > Changelog since v1
> > > o Fix unsafe RT locking scheme
> > > o Use spin_trylock on UP PREEMPT_RT
> > >
> > > This series has the same intent as Nicolas' series "mm/page_alloc: Remote
> > > per-cpu lists drain support" -- avoid interference of a high priority
> > > task due to a workqueue item draining per-cpu page lists. While many
> > > workloads can tolerate a brief interruption, it may be cause a real-time
> > > task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,
> > > the draining in non-deterministic.
> > >
> > > Currently an IRQ-safe local_lock protects the page allocator per-cpu lists.
> > > The local_lock on its own prevents migration and the IRQ disabling protects
> > > from corruption due to an interrupt arriving while a page allocation is
> > > in progress. The locking is inherently unsafe for remote access unless
> > > the CPU is hot-removed.
> > >
> > > This series adjusts the locking. A spinlock is added to struct
> > > per_cpu_pages to protect the list contents while local_lock_irq continues
> > > to prevent migration and IRQ reentry. This allows a remote CPU to safely
> > > drain a remote per-cpu list.
> > >
> > > This series is a partial series. Follow-on work should allow the
> > > local_irq_save to be converted to a local_irq to avoid IRQs being
> > > disabled/enabled in most cases. Consequently, there are some TODO comments
> > > highlighting the places that would change if local_irq was used. However,
> > > there are enough corner cases that it deserves a series on its own
> > > separated by one kernel release and the priority right now is to avoid
> > > interference of high priority tasks.
> >
> > Reverting the whole series fixed an issue that offlining a memory
> > section blocking for hours on today's linux-next tree.
> >
> > __wait_rcu_gp
> > synchronize_rcu at kernel/rcu/tree.c:3915
> > lru_cache_disable at mm/swap.c:886
> > __alloc_contig_migrate_range at mm/page_alloc.c:9078
> > isolate_single_pageblock at mm/page_isolation.c:405
> > start_isolate_page_range
> > offline_pages
> > memory_subsys_offline
> > device_offline
> > online_store
> > dev_attr_store
> > sysfs_kf_write
> > kernfs_fop_write_iter
> > new_sync_write
> > vfs_write
> > ksys_write
> > __arm64_sys_write
> > invoke_syscall
> > el0_svc_common.constprop.0
> > do_el0_svc
> > el0_svc
> > el0t_64_sync_handler
> > el0t_64_sync
> >
> > For full disclosure, I have also reverted the commit 0d523026abd4
> > ("mm/page_alloc: fix tracepoint mm_page_alloc_zone_locked()"), so the
> > series can be reverted cleanly. But, I can't see how the commit
> > 0d523026abd4 could cause this issue at all.
>
> This is halting in __lru_add_drain_all where it calls synchronize_rcu
> before the drain even happens. It's also an LRU drain and not PCP which
> is what the series affects and the allocator doesn't use rcu. In a KVM
> machine, I can do
>
> $ for BANK in `(for i in {1..20}; do echo $((RANDOM%416)); done) | sort -n | uniq`; do BEFORE=`cat /sys/devices/system/memory/memory$BANK/online`; echo 0 > /sys/devices/system/memory/memory$BANK/online; AFTER=`cat /sys/devices/system/memory/memory$BANK/online`; printf "%4d %d -> %d\n" $BANK $BEFORE $AFTER; done
> 3 1 -> 0
> 57 1 -> 0
> 74 1 -> 0
> 93 1 -> 0
> 101 1 -> 0
> 128 1 -> 0
> 133 1 -> 0
> 199 1 -> 0
> 223 1 -> 0
> 225 1 -> 0
> 229 1 -> 0
> 243 1 -> 0
> 263 1 -> 0
> 300 1 -> 0
> 309 1 -> 0
> 329 1 -> 0
> 355 1 -> 0
> 365 1 -> 0
> 372 1 -> 0
> 383 1 -> 0
>
> It offlines 20 sections although after several attempts free -m starts
> reporting negative used memory so there is a bug of some description.
> How are you testing this exactly? Is it every time or intermittent? Are
> you confident that reverting the series makes the problem go away?
Cc'ing Paul. Either reverting this series or Paul's 3 patches below from
today's linux-next tree fixed the issue.
ca52639daa5b rcu-tasks: Drive synchronous grace periods from calling task
89ad98e93ce8 rcu-tasks: Move synchronize_rcu_tasks_generic() down
0d90e7225fb1 rcu-tasks: Split rcu_tasks_one_gp() from rcu_tasks_kthread()
It was reproduced by running this script below on an arm64 server. I can
reproduce it every time within 5 attempts. I noticed that when it happens,
we have a few rcu kthreads all are stuck in this line,
rcuwait_wait_event(&rtp->cbs_wait,
(needgpcb = rcu_tasks_need_gpcb(rtp)),
TASK_IDLE);
rcu_tasks_kthread
rcu_tasks_rude_kthread
[rcu_tasks_trace_kthread
#!/usr/bin/env python3
# SPDX-License-Identifier: GPL-2.0
import os
import re
import subprocess
def mem_iter():
base_dir = '/sys/devices/system/memory/'
for curr_dir in os.listdir(base_dir):
if re.match(r'memory\d+', curr_dir):
yield base_dir + curr_dir
if __name__ == '__main__':
print('- Try to remove each memory section and then add it back.')
for mem_dir in mem_iter():
status = f'{mem_dir}/online'
if open(status).read().rstrip() == '1':
# This could expectedly fail due to many reasons.
section = os.path.basename(mem_dir)
print(f'- Try to remove {section}.')
proc = subprocess.run([f'echo 0 | sudo tee {status}'], shell=True)
if proc.returncode == 0:
print(f'- Try to add {section}.')
subprocess.check_call([f'echo 1 | sudo tee {status}'], shell=True)
