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Re: XFS Syncd

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: XFS Syncd
From: Shrinand Javadekar <shrinand@xxxxxxxxxxxxxx>
Date: Fri, 5 Jun 2015 10:31:11 -0700
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <CABppvi5wNHA18nY497YK0fj4GU+3VLC8VntQ1JKNDR6EErrKxg@xxxxxxxxxxxxxx>
References: <CABppvi4e_xEMY7tDHtEo6miZcN2AZ-mFMHXKaUS0hfpx6AMt0w@xxxxxxxxxxxxxx> <20150410072100.GL13731@dastard> <CABppvi437S9e+DEFOi6ECPu8=AnEK0V=5rRmU5Of1_XtWiQbfA@xxxxxxxxxxxxxx> <20150410131245.GK15810@dastard> <CABppvi68E6n+pr6X8TMOBhicVB4mrJbyyvm89r56rRVqSjf1Zg@xxxxxxxxxxxxxx> <20150603035719.GO24666@dastard> <CABppvi4AzQyaUm25_ombXR0Om04mUcHKtFd0ug_iKRxqa+NsOg@xxxxxxxxxxxxxx> <20150604003546.GS24666@dastard> <20150604012530.GG9143@dastard> <20150604020339.GI9143@dastard> <20150604062337.GK9143@dastard> <CABppvi5wNHA18nY497YK0fj4GU+3VLC8VntQ1JKNDR6EErrKxg@xxxxxxxxxxxxxx>
The file xfs_buf.c seems to have gone through a few revisions. I tried
to understand the code and make the changes in the 3.16.0 kernel but
it didn't quite work out. XFS crashed while unmounting the disks.

On Thu, Jun 4, 2015 at 5:59 PM, Shrinand Javadekar
<shrinand@xxxxxxxxxxxxxx> wrote:
> Dave,
>
> I believe this code is slightly different from the one I have (kernel
> v3.16.0). Can you give me a patch for kernel v3.16.0? I have a working
> setup to try this out.
>
> http://lxr.free-electrons.com/source/fs/xfs/xfs_buf.c?v=3.16
>
> Thanks in advance.
> -Shri
>
>
> On Wed, Jun 3, 2015 at 11:23 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> On Thu, Jun 04, 2015 at 12:03:39PM +1000, Dave Chinner wrote:
>>> Fixing this requires a tweak to the algorithm in
>>> __xfs_buf_delwri_submit() so that we don't lock an entire list of
>>> thousands of IOs before starting submission. In the mean time,
>>> reducing the number of AGs will reduce the impact of this because
>>> the delayed write submission code will skip buffers that are already
>>> locked or pinned in memory, and hence an AG under modification at
>>> the time submission occurs will be skipped by the delwri code.
>>
>> You might like to try the patch below on a test machine to see if
>> it helps with the problem.
>>
>> Cheers,
>>
>> Dave.
>> --
>> Dave Chinner
>> david@xxxxxxxxxxxxx
>>
>> xfs: reduce lock hold times in buffer writeback
>>
>> From: Dave Chinner <dchinner@xxxxxxxxxx>
>>
>> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
>> ---
>>  fs/xfs/xfs_buf.c | 80 
>> ++++++++++++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 61 insertions(+), 19 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
>> index bbe4e9e..8d2cc36 100644
>> --- a/fs/xfs/xfs_buf.c
>> +++ b/fs/xfs/xfs_buf.c
>> @@ -1768,15 +1768,63 @@ xfs_buf_cmp(
>>         return 0;
>>  }
>>
>> +static void
>> +xfs_buf_delwri_submit_buffers(
>> +       struct list_head        *buffer_list,
>> +       struct list_head        *io_list,
>> +       bool                    wait)
>> +{
>> +       struct xfs_buf          *bp, *n;
>> +       struct blk_plug         plug;
>> +
>> +       blk_start_plug(&plug);
>> +       list_for_each_entry_safe(bp, n, buffer_list, b_list) {
>> +               bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_ASYNC |
>> +                                 XBF_WRITE_FAIL);
>> +               bp->b_flags |= XBF_WRITE | XBF_ASYNC;
>> +
>> +               /*
>> +                * We do all IO submission async. This means if we need
>> +                * to wait for IO completion we need to take an extra
>> +                * reference so the buffer is still valid on the other
>> +                * side. We need to move the buffer onto the io_list
>> +                * at this point so the caller can still access it.
>> +                */
>> +               if (wait) {
>> +                       xfs_buf_hold(bp);
>> +                       list_move_tail(&bp->b_list, io_list);
>> +               } else
>> +                       list_del_init(&bp->b_list);
>> +
>> +               xfs_buf_submit(bp);
>> +       }
>> +       blk_finish_plug(&plug);
>> +}
>> +
>> +/*
>> + * submit buffers for write.
>> + *
>> + * When we have a large buffer list, we do not want to hold all the buffers
>> + * locked while we block on the request queue waiting for IO dispatch. To 
>> avoid
>> + * this problem, we lock and submit buffers in groups of 50, thereby 
>> minimising
>> + * the lock hold times for lists which may contain thousands of objects.
>> + *
>> + * To do this, we sort the buffer list before we walk the list to lock and
>> + * submit buffers, and we plug and unplug around each group of buffers we
>> + * submit.
>> + */
>>  static int
>>  __xfs_buf_delwri_submit(
>>         struct list_head        *buffer_list,
>>         struct list_head        *io_list,
>>         bool                    wait)
>>  {
>> -       struct blk_plug         plug;
>>         struct xfs_buf          *bp, *n;
>> +       LIST_HEAD               (submit_list);
>>         int                     pinned = 0;
>> +       int                     count = 0;
>> +
>> +       list_sort(NULL, buffer_list, xfs_buf_cmp);
>>
>>         list_for_each_entry_safe(bp, n, buffer_list, b_list) {
>>                 if (!wait) {
>> @@ -1802,30 +1850,24 @@ __xfs_buf_delwri_submit(
>>                         continue;
>>                 }
>>
>> -               list_move_tail(&bp->b_list, io_list);
>> +               list_move_tail(&bp->b_list, &submit_list);
>>                 trace_xfs_buf_delwri_split(bp, _RET_IP_);
>> -       }
>> -
>> -       list_sort(NULL, io_list, xfs_buf_cmp);
>> -
>> -       blk_start_plug(&plug);
>> -       list_for_each_entry_safe(bp, n, io_list, b_list) {
>> -               bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_ASYNC | XBF_WRITE_FAIL);
>> -               bp->b_flags |= XBF_WRITE | XBF_ASYNC;
>>
>>                 /*
>> -                * we do all Io submission async. This means if we need to 
>> wait
>> -                * for IO completion we need to take an extra reference so 
>> the
>> -                * buffer is still valid on the other side.
>> +                * We do small batches of  IO submission to minimise lock 
>> hold
>> +                * time and unnecessary writeback of buffers that are hot and
>> +                * would otherwise be relogged and hence not require 
>> immediate
>> +                * writeback.
>>                  */
>> -               if (wait)
>> -                       xfs_buf_hold(bp);
>> -               else
>> -                       list_del_init(&bp->b_list);
>> +               if (count++ < 50)
>> +                       continue;
>>
>> -               xfs_buf_submit(bp);
>> +               xfs_buf_delwri_submit_buffers(&submit_list, io_list, wait);
>> +               count = 0;
>>         }
>> -       blk_finish_plug(&plug);
>> +
>> +       if (!list_empty(&submit_list))
>> +               xfs_buf_delwri_submit_buffers(&submit_list, io_list, wait);
>>
>>         return pinned;
>>  }

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