> With the multi block allocator when we don't have prealloc space we discard
> the existing preallocaltion data and try to rebuild the buddy cache. While
> discarding the loop through the group specific prealloc list. If we find any
> particular prealloc space being used we mark the space busy. If we are not
> able to find enough free space and if we have any prealloc space busy we loop
> back again. With non preempted kernel this tight loop resulted in watchdog
> timer triggering soft lockup warning.
>
>
> Whe we are allocation the block we search the prealloc list and mark the
> prealloc space used via incrementing pa_count value. One after succesffuly
> allocating the block we need to update the block bitmap and this could
> actually involved a disk io if the bitmap need to read from the disk. This
> actually cause prealloc space to be marked used for quiet a long time. This
> inturn results in the discard logic going on tight loop resulting in watchdog
> timer triggering soft lockup warning.
>
> Signed-off-by: Aneesh Kumar K.V <[EMAIL PROTECTED]>
> ---
> fs/ext4/mballoc.c | 12 +++---------
> 1 files changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 844765c..cbc8143 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3729,7 +3729,7 @@ static int ext4_mb_discard_group_preallocations(struct
> super_block *sb,
> struct list_head list;
> struct ext4_buddy e4b;
> int err;
> - int busy;
> + int busy = 0;
> int free = 0;
>
> mb_debug("discard preallocation for group %lu\n", group);
> @@ -3754,20 +3754,12 @@ static int
> ext4_mb_discard_group_preallocations(struct super_block *sb,
> INIT_LIST_HEAD(&list);
>
> repeat:
> - busy = 0;
> ext4_lock_group(sb, group);
> list_for_each_entry_safe(pa, tmp,
> &grp->bb_prealloc_list, pa_group_list) {
> spin_lock(&pa->pa_lock);
> if (atomic_read(&pa->pa_count)) {
> spin_unlock(&pa->pa_lock);
> - /* FIXME!!
> - * It is quiet natural to have the pa being
> - * used on other cpus when we are trying free
> - * space
> - printk(KERN_ERR "uh! busy PA\n");
> - dump_stack();
> - */
> busy = 1;
> continue;
> }
> @@ -3790,7 +3782,9 @@ repeat:
>
> /* if we still need more blocks and some PAs were used, try again */
> if (free < needed && busy) {
> + busy = 0;
> ext4_unlock_group(sb, group);
> + schedule_timeout(HZ);
> goto repeat;
> }
Hmm, wouldn't just schedule() be enough here? That would give a good
chance to other processes to proceed and we would avoid this artificial
wait of 1s which is quite ugly IMO.
Honza
--
Jan Kara <[EMAIL PROTECTED]>
SuSE CR Labs
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