Re: [PATCH 18/23] xfs: collapse AG selection for inode allocation

Brian Foster
2021-05-27T11:20:02Z

On Wed, May 19, 2021 at 11:20:57AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> xfs_dialloc_select_ag() does a lot of repetitive work. It first
> calls xfs_ialloc_ag_select() to select the AG to start allocation
> attempts in, which can do up to two entire loops across the perags
> that inodes can be allocated in. This is simply checking if there is
> spce available to allocate inodes in an AG, and it returns when it
> finds the first candidate AG.
>
> xfs_dialloc_select_ag() then does it's own iterative walk across
> all the perags locking the AGIs and trying to allocate inodes from
> the locked AG. It also doesn't limit the search to mp->m_maxagi,
> so it will walk all AGs whether they can allocate inodes or not.
>
> Hence if we are really low on inodes, we could do almost 3 entire
> walks across the whole perag range before we find an allocation
> group we can allocate inodes in or report ENOSPC.
>
> Because xfs_ialloc_ag_select() returns on the first candidate AG it
> finds, we can simply do these checks directly in
> xfs_dialloc_select_ag() before we lock and try to allocate inodes.
> This reduces the inode allocation pass down to 2 perag sweeps at
> most - one for aligned inode cluster allocation and if we can't
> allocate full, aligned inode clusters anywhere we'll do another pass
> trying to do sparse inode cluster allocation.
>
> This also removes a big chunk of duplicate code.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_ialloc.c | 220 +++++++++++++------------------------
> 1 file changed, 74 insertions(+), 146 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 872591e8f5cb..3acecdbe51e4 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -899,139 +899,6 @@ xfs_ialloc_ag_alloc(
> return 0;
> }
>
> -STATIC xfs_agnumber_t
> -xfs_ialloc_next_ag(
> - xfs_mount_t *mp)
> -{
> - xfs_agnumber_t agno;
> -
> - spin_lock(&mp->m_agirotor_lock);
> - agno = mp->m_agirotor;
> - if (++mp->m_agirotor >= mp->m_maxagi)
> - mp->m_agirotor = 0;
> - spin_unlock(&mp->m_agirotor_lock);
> -
> - return agno;
> -}
> -
> -/*
> - * Select an allocation group to look for a free inode in, based on the parent
> - * inode and the mode. Return the allocation group buffer.
> - */
> -STATIC xfs_agnumber_t
> -xfs_ialloc_ag_select(
> - xfs_trans_t *tp, /* transaction pointer */
> - xfs_ino_t parent, /* parent directory inode number */
> - umode_t mode) /* bits set to indicate file type */
> -{
> - xfs_agnumber_t agcount; /* number of ag's in the filesystem */
> - xfs_agnumber_t agno; /* current ag number */
> - int flags; /* alloc buffer locking flags */
> - xfs_extlen_t ineed; /* blocks needed for inode allocation */
> - xfs_extlen_t longest = 0; /* longest extent available */
> - xfs_mount_t *mp; /* mount point structure */
> - int needspace; /* file mode implies space allocated */
> - xfs_perag_t *pag; /* per allocation group data */
> - xfs_agnumber_t pagno; /* parent (starting) ag number */
> - int error;
> -
> - /*
> - * Files of these types need at least one block if length > 0
> - * (and they won't fit in the inode, but that's hard to figure out).
> - */
> - needspace = S_ISDIR(mode) || S_ISREG(mode) || S_ISLNK(mode);
> - mp = tp->t_mountp;
> - agcount = mp->m_maxagi;
> - if (S_ISDIR(mode))
> - pagno = xfs_ialloc_next_ag(mp);
> - else {
> - pagno = XFS_INO_TO_AGNO(mp, parent);
> - if (pagno >= agcount)
> - pagno = 0;
> - }
> -
> - ASSERT(pagno < agcount);
> -
> - /*
> - * Loop through allocation groups, looking for one with a little
> - * free space in it. Note we don't look for free inodes, exactly.
> - * Instead, we include whether there is a need to allocate inodes
> - * to mean that blocks must be allocated for them,
> - * if none are currently free.
> - */
> - agno = pagno;
> - flags = XFS_ALLOC_FLAG_TRYLOCK;
> - for (;;) {
> - pag = xfs_perag_get(mp, agno);
> - if (!pag->pagi_inodeok) {
> - xfs_ialloc_next_ag(mp);
> - goto nextag;
> - }
> -
> - if (!pag->pagi_init) {
> - error = xfs_ialloc_pagi_init(mp, tp, agno);
> - if (error)
> - goto nextag;
> - }
> -
> - if (pag->pagi_freecount) {
> - xfs_perag_put(pag);
> - return agno;
> - }
> -
> - if (!pag->pagf_init) {
> - error = xfs_alloc_pagf_init(mp, tp, agno, flags);
> - if (error)
> - goto nextag;
> - }
> -
> - /*
> - * Check that there is enough free space for the file plus a
> - * chunk of inodes if we need to allocate some. If this is the
> - * first pass across the AGs, take into account the potential
> - * space needed for alignment of inode chunks when checking the
> - * longest contiguous free space in the AG - this prevents us
> - * from getting ENOSPC because we have free space larger than
> - * ialloc_blks but alignment constraints prevent us from using
> - * it.
> - *
> - * If we can't find an AG with space for full alignment slack to
> - * be taken into account, we must be near ENOSPC in all AGs.
> - * Hence we don't include alignment for the second pass and so
> - * if we fail allocation due to alignment issues then it is most
> - * likely a real ENOSPC condition.
> - */
> - ineed = M_IGEO(mp)->ialloc_min_blks;
> - if (flags && ineed > 1)
> - ineed += M_IGEO(mp)->cluster_align;
> - longest = pag->pagf_longest;
> - if (!longest)
> - longest = pag->pagf_flcount > 0;
> -
> - if (pag->pagf_freeblks >= needspace + ineed &&
> - longest >= ineed) {
> - xfs_perag_put(pag);
> - return agno;
> - }
> -nextag:
> - xfs_perag_put(pag);
> - /*
> - * No point in iterating over the rest, if we're shutting
> - * down.
> - */
> - if (XFS_FORCED_SHUTDOWN(mp))
> - return NULLAGNUMBER;
> - agno++;
> - if (agno >= agcount)
> - agno = 0;
> - if (agno == pagno) {
> - if (flags == 0)
> - return NULLAGNUMBER;
> - flags = 0;
> - }
> - }
> -}
> -
> /*
> * Try to retrieve the next record to the left/right from the current one.
> */
> @@ -1708,6 +1575,21 @@ xfs_dialloc_roll(
> return 0;
> }
>
> +STATIC xfs_agnumber_t
> +xfs_ialloc_next_ag(
> + xfs_mount_t *mp)
> +{
> + xfs_agnumber_t agno;
> +
> + spin_lock(&mp->m_agirotor_lock);
> + agno = mp->m_agirotor;
> + if (++mp->m_agirotor >= mp->m_maxagi)
> + mp->m_agirotor = 0;
> + spin_unlock(&mp->m_agirotor_lock);
> +
> + return agno;
> +}
> +
> /*
> * Select and prepare an AG for inode allocation.
> *
> @@ -1734,16 +1616,24 @@ xfs_dialloc_select_ag(
> struct xfs_perag *pag;
> struct xfs_ino_geometry *igeo = M_IGEO(mp);
> bool okalloc = true;
> + int needspace;
> + int flags;
>
> *IO_agbp = NULL;
>
> /*
> - * We do not have an agbp, so select an initial allocation
> - * group for inode allocation.
> + * Directories, symlinks, and regular files frequently allocate at least
> + * one block, so factor that potential expansion when we examine whether
> + * an AG has enough space for file creation.
> */
> - start_agno = xfs_ialloc_ag_select(*tpp, parent, mode);
> - if (start_agno == NULLAGNUMBER)
> - return -ENOSPC;
> + needspace = S_ISDIR(mode) || S_ISREG(mode) || S_ISLNK(mode);
> + if (S_ISDIR(mode))
> + start_agno = xfs_ialloc_next_ag(mp);
> + else {
> + start_agno = XFS_INO_TO_AGNO(mp, parent);
> + if (start_agno >= mp->m_maxagi)
> + start_agno = 0;
> + }
>
> /*
> * If we have already hit the ceiling of inode blocks then clear
> @@ -1765,12 +1655,14 @@ xfs_dialloc_select_ag(
> * allocation groups upward, wrapping at the end.
> */
> agno = start_agno;
> + flags = XFS_ALLOC_FLAG_TRYLOCK;
> for (;;) {
> + xfs_extlen_t ineed;
> + xfs_extlen_t longest = 0;
> +
> pag = xfs_perag_get(mp, agno);
> - if (!pag->pagi_inodeok) {
> - xfs_ialloc_next_ag(mp);
> + if (!pag->pagi_inodeok)
> goto nextag;
> - }
>
> if (!pag->pagi_init) {
> error = xfs_ialloc_pagi_init(mp, *tpp, agno);
> @@ -1778,10 +1670,39 @@ xfs_dialloc_select_ag(
> break;
> }
>
> + if (!pag->pagi_freecount && !okalloc)
> + goto nextag;
> +
> + if (!pag->pagf_init) {
> + error = xfs_alloc_pagf_init(mp, *tpp, agno, flags);
> + if (error)
> + goto nextag;
> + }
> +
> /*
> - * Do a first racy fast path check if this AG is usable.
> + * Check that there is enough free space for the file plus a
> + * chunk of inodes if we need to allocate some. If this is the
> + * first pass across the AGs, take into account the potential
> + * space needed for alignment of inode chunks when checking the
> + * longest contiguous free space in the AG - this prevents us
> + * from getting ENOSPC because we have free space larger than
> + * ialloc_blks but alignment constraints prevent us from using
> + * it.
> + *
> + * If we can't find an AG with space for full alignment slack to
> + * be taken into account, we must be near ENOSPC in all AGs.
> + * Hence we don't include alignment for the second pass and so
> + * if we fail allocation due to alignment issues then it is most
> + * likely a real ENOSPC condition.
> */
> - if (!pag->pagi_freecount && !okalloc)
> + ineed = M_IGEO(mp)->ialloc_min_blks;
> + if (flags && ineed > 1)
> + ineed += M_IGEO(mp)->cluster_align;
> + longest = pag->pagf_longest;
> + if (!longest)
> + longest = pag->pagf_flcount > 0;
> +
> + if (pag->pagf_freeblks < needspace + ineed || longest < ineed)
> goto nextag;

Similar to last time.. it looks like this skips an AG based on free
space even if there are allocated inode chunks with free inodes.

Brian

>
> /*
> @@ -1823,10 +1744,17 @@ xfs_dialloc_select_ag(
> nextag_relse_buffer:
> xfs_trans_brelse(*tpp, agbp);
> nextag:
> - if (++agno == mp->m_sb.sb_agcount)
> - agno = 0;
> - if (agno == start_agno)
> + if (XFS_FORCED_SHUTDOWN(mp)) {
> + error = -EFSCORRUPTED;
> break;
> + }
> + if (++agno == mp->m_maxagi)
> + agno = 0;
> + if (agno == start_agno) {
> + if (!flags)
> + break;
> + flags = 0;
> + }
> xfs_perag_put(pag);
> }
>
> --
> 2.31.1
>