linux-kernel - Re: Fw: RCU detected CPU 1 stall (t=4295904002/751 jiffies)Pid:902, comm: md1

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Date:	Thu, 21 May 2009 11:50:13 +0200
From:	"Janos Haar" <janos.haar@...center.hu>
To:	"Neil Brown" <neilb@...e.de>
Cc:	<paulmck@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: Fw: RCU detected CPU 1 stall (t=4295904002/751 jiffies)Pid:902, comm: md1_raid5

Hello,

Just a brief note preliminary:
I am still compiling my kernel (without the patch) for preemtion enabled, 
the load is 2.00.
I have removed the bitmap from my array, and thel load falls down to 1.00, 
the delays are gone.
(The actual kernek not supports RCU debugging, so there is no error 
message.)

I need to run now, i will try the patch later....
Thanks a lot,

Janos Haar

[root@...diator-afth1 ~]# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
[multipath] [faulty]
md1 : active raid6 sdd2[3] sdc2[1] sdb2[0] sda2[2]
      19550976 blocks level 6, 16k chunk, algorithm 2 [4/4] [UUUU]

md0 : active raid1 sdd1[3] sdc1[1] sdb1[0] sda1[2]
      104320 blocks [4/4] [UUUU]

unused devices: <none>
[root@...diator-afth1 ~]#

The internal bitmap was in md1.

----- Original Message ----- 
From: "Neil Brown" <neilb@...e.de>
To: <paulmck@...ux.vnet.ibm.com>
Cc: "Janos Haar" <janos.haar@...center.hu>; <linux-kernel@...r.kernel.org>
Sent: Thursday, May 21, 2009 8:50 AM
Subject: Re: Fw: RCU detected CPU 1 stall (t=4295904002/751 jiffies)Pid:902, 
comm: md1_raid5


> On Wednesday May 20, paulmck@...ux.vnet.ibm.com wrote:
>> On Thu, May 21, 2009 at 06:46:15AM +0200, Janos Haar wrote:
>> > Paul,
>> >
>> > Thank you for your attention.
>> > Yes, the PC makes 2-3 second "pauses" and drop this message again and
>> > again.
>> > If i remove the RCU debugging, the message disappears, but the pauses 
>> > still
>> > here, and makes 2-3 load on the idle system.
>> > Can i do something?
>> > You suggest to use PREEMPT? (This is a server.)
>>
>> One possibility is that the lock that bitmap_daemon_work() acquires is
>> being held for too long.  Another possibility is the list traversal in
>> md_check_recovery() that might loop for a long time if the list were
>> excessively long or could be temporarily tied in a knot.
>>
>> Neil, thoughts?
>>
>
> I would be surprised if any of these things take as long as 3 seconds
> (or even 1 second) but I cannot completely rule it out.
>
> I assume that you mean 3 seconds of continuous running with no
> sleeping, so it cannot be a slow kmalloc that is causing the delay?
>
> bitmap_daemon_work is the most likely candidate as bitmap->chunks
> can be very large (thousands, probably not millions though).
> Taking and dropping the lock every time around that loop doesn't
> really make much sense, does it....
> And it looks like it could actually be optimised quite a bit to skip a
> lot of the iterations in most cases - there are two places where we
> can accelerate 'j' quite a lot.
>
> Janos: Can you try this and see if it makes a difference?
> Thanks.
>
> NeilBrown
>
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 47c68bc..56df1ce 100644
> --- a/drivers/md/bitmap.c
> +++ b/drivers/md/bitmap.c
> @@ -1097,14 +1097,12 @@ void bitmap_daemon_work(struct bitmap *bitmap)
>  }
>  bitmap->allclean = 1;
>
> + spin_lock_irqsave(&bitmap->lock, flags);
>  for (j = 0; j < bitmap->chunks; j++) {
>  bitmap_counter_t *bmc;
> - spin_lock_irqsave(&bitmap->lock, flags);
> - if (!bitmap->filemap) {
> + if (!bitmap->filemap)
>  /* error or shutdown */
> - spin_unlock_irqrestore(&bitmap->lock, flags);
>  break;
> - }
>
>  page = filemap_get_page(bitmap, j);
>
> @@ -1121,6 +1119,8 @@ void bitmap_daemon_work(struct bitmap *bitmap)
>  write_page(bitmap, page, 0);
>  bitmap->allclean = 0;
>  }
> + spin_lock_irqsave(&bitmap->lock, flags);
> + j |= (PAGE_BITS - 1);
>  continue;
>  }
>
> @@ -1181,9 +1181,10 @@ void bitmap_daemon_work(struct bitmap *bitmap)
>  ext2_clear_bit(file_page_offset(j), paddr);
>  kunmap_atomic(paddr, KM_USER0);
>  }
> - }
> - spin_unlock_irqrestore(&bitmap->lock, flags);
> + } else
> + j |= PAGE_COUNTER_MASK;
>  }
> + spin_unlock_irqrestore(&bitmap->lock, flags);
>
>  /* now sync the final page */
>  if (lastpage != NULL) {
> --
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