linux-ext4 - Re: [PATCH] fix softlockups in ext2/3 when trying to allocate blocks

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Date:	Mon, 20 Jul 2009 23:37:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-ext4@...r.kernel.org, emcnabb@...hat.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Mingming Cao <cmm@...ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] fix softlockups in ext2/3 when trying to allocate
 blocks

On Mon, 6 Jul 2009 15:47:39 -0400 Josef Bacik <josef@...hat.com> wrote:

> This isn't a huge deal, but using a big beefy box with more CPUs than what is
> sane, you can get a nice flood of softlockup messages when running heavy
> multi-threaded io tests on ext2/3.  The processors compete for blocks from the
> allocator, so they will loop quite a bit trying to get their allocation.  This
> patch simply makes sure that we reschedule if need be.  This made the softlockup
> messages disappear whereas before they happened almost immediately.  Thanks,

The softlockup threshold is 60 seconds.  For the kernel to spend 60
seconds continuous CPU time in the filesystem is very bad behaviour, and
adding a rescheduling point doesn't fix that!

> Tested-by: Evan McNabb <emcnabb@...hat.com>
> Signed-off-by: Josef Bacik <josef@...hat.com>
> ---
>  fs/ext2/balloc.c |    1 +
>  fs/ext3/balloc.c |    2 ++
>  2 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
> index 7f8d2e5..17dd55f 100644
> --- a/fs/ext2/balloc.c
> +++ b/fs/ext2/balloc.c
> @@ -1176,6 +1176,7 @@ ext2_try_to_allocate_with_rsv(struct super_block *sb, unsigned int group,
>  			break;				/* succeed */
>  		}
>  		num = *count;
> +		cond_resched();
>  	}
>  	return ret;
>  }
> diff --git a/fs/ext3/balloc.c b/fs/ext3/balloc.c
> index 27967f9..cffc8cd 100644
> --- a/fs/ext3/balloc.c
> +++ b/fs/ext3/balloc.c
> @@ -735,6 +735,7 @@ bitmap_search_next_usable_block(ext3_grpblk_t start, struct buffer_head *bh,
>  	struct journal_head *jh = bh2jh(bh);
>  
>  	while (start < maxblocks) {
> +		cond_resched();
>  		next = ext3_find_next_zero_bit(bh->b_data, maxblocks, start);
>  		if (next >= maxblocks)
>  			return -1;
> @@ -1391,6 +1392,7 @@ ext3_try_to_allocate_with_rsv(struct super_block *sb, handle_t *handle,
>  			break;				/* succeed */
>  		}
>  		num = *count;
> +		cond_resched();
>  	}
>  out:
>  	if (ret >= 0) {

I worry that something has gone wrong with the reservations code.  The
filesystem _should_ be able to find a free block without any contention
from other CPUs, because there's a range of blocks reserved for this
inode's allocation attempts.

Unless the workload has a lot of threads writing to the _same_ file. 
If it does that then yes, we'll have lots of CPUs contenting for blocks
within that inode's reservation window.  Tell us about the workload please.

But that shouldn't be happening either because all those write()ing
threads will be serialised by i_mutex.

So I don't know what's happening here.  Possibly a better fix would be
to add a lock rather than leaving the contention in place and hiding
it.  Even better would be to understand why the contention is happening
and prevent that.

Thanks.
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