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?
--
Mel Gorman
SUSE Labs
