Re: unexpected -ENOMEM from percpu_counter_init()

[Wang Yugui <wangyugui@e16-tech.com>]

Hi, Dennis Zhou, Vlastimil Babka, Filipe Manana

The root reason of this problem maybe the design of
'memalloc_nofs_restore()/memalloc_nofs_save()'.

When some job such as memory pre-alloc and reclaim is needed,  that is
done in a workqueue now.

This is a problem for high-load and over-load. In that case, we need to
do these job in current task/process, so that current task/process will
be blocked until necessary job is done.

If we let these job in done in a workqueue, and current task/process is
not blocked, that means failure is very near, and then we can not work
stable in high-load and over-load.

For high-load and over-load, failure is not expected,  we expect some
job be blocked well.

> > Percpu does do this via a workqueue item. The issue is in v5.9 we
> > introduced 2 types of chunks. However, the free float page number was
> > for the total. So even if 1 chunk type dropped below, the other chunk
> > type might have enough pages. I'm queuing this for 5.12 and will send it
> > out assuming it does fix your problem.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2021/04/09

> +CC btrfs
> 
> On 4/1/21 12:51 PM, Wang Yugui wrote:
> > Hi,
> > 
> > an unexpected -ENOMEM from percpu_counter_init() happened when xfstest 
> > with kernel 5.11.10 and 5.10.27
> 
> Is there a dmesg log showing allocation failure or something?
> 
> > direct caller:
> > int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
> > {
> >     int ret;
> > 
> >     ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> >     if (ret)
> >         return ret;
> > 
> >     atomic_set(&lock->readers, 0);
> >     init_waitqueue_head(&lock->pending_readers);
> >     init_waitqueue_head(&lock->pending_writers);
> > 
> >     return 0;
> > }
> > 
> > upper caller:
> >     nofs_flag = memalloc_nofs_save();
> >     ret = btrfs_drew_lock_init(&root->snapshot_lock);
> >     memalloc_nofs_restore(nofs_flag);
> >     if (ret == -ENOMEM) printk("ENOMEM btrfs_drew_lock_init\n");
> >     if (ret)
> >         goto fail;
> > 
> > The hardware of this server:
> > CPU:  Xeon(R) CPU E5-2660 v2(10 core)  *2
> > memory:  192G, no swap
> > 
> > Only one xfstests job is running in this server, and about 7% of memory
> > is used.
> > 
> > Any advice please.
> > 
> > Best Regards
> > Wang Yugui (wangyugui@e16-tech.com)
> > 2021/04/01
> > 
> >