xfs: reduce lock hold times in buffer writeback
When we have a lot of metadata to flush from the AIL, the buffer list can get very long. The current submission code tries to batch submission to optimise IO order of the metadata (i.e. ascending block order) to maximise block layer merging or IO to adjacent metadata blocks. Unfortunately, the method used can result in long lock times occurring as buffers locked early on in the buffer list might not be dispatched until the end of the IO licst processing. This is because sorting does not occur util after the buffer list has been processed and the buffers that are going to be submitted are locked. Hence when the buffer list is several thousand buffers long, the lock hold times before IO dispatch can be significant. To fix this, sort the buffer list before we start trying to lock and submit buffers. This means we can now submit buffers immediately after they are locked, allowing merging to occur immediately on the plug and dispatch to occur as quickly as possible. This means there is minimal delay between locking the buffer and IO submission occuring, hence reducing the worst case lock hold times seen during delayed write buffer IO submission signficantly. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>
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@ -1774,18 +1774,33 @@ xfs_buf_cmp(
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return 0;
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}
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/*
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* submit buffers for write.
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*
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* When we have a large buffer list, we do not want to hold all the buffers
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* locked while we block on the request queue waiting for IO dispatch. To avoid
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* this problem, we lock and submit buffers in groups of 50, thereby minimising
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* the lock hold times for lists which may contain thousands of objects.
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*
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* To do this, we sort the buffer list before we walk the list to lock and
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* submit buffers, and we plug and unplug around each group of buffers we
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* submit.
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*/
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static int
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__xfs_buf_delwri_submit(
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xfs_buf_delwri_submit_buffers(
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struct list_head *buffer_list,
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struct list_head *io_list,
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bool wait)
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struct list_head *wait_list)
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{
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struct blk_plug plug;
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struct xfs_buf *bp, *n;
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LIST_HEAD (submit_list);
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int pinned = 0;
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struct blk_plug plug;
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list_sort(NULL, buffer_list, xfs_buf_cmp);
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blk_start_plug(&plug);
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list_for_each_entry_safe(bp, n, buffer_list, b_list) {
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if (!wait) {
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if (!wait_list) {
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if (xfs_buf_ispinned(bp)) {
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pinned++;
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continue;
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@ -1808,25 +1823,21 @@ __xfs_buf_delwri_submit(
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continue;
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}
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list_move_tail(&bp->b_list, io_list);
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trace_xfs_buf_delwri_split(bp, _RET_IP_);
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}
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list_sort(NULL, io_list, xfs_buf_cmp);
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blk_start_plug(&plug);
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list_for_each_entry_safe(bp, n, io_list, b_list) {
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bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_ASYNC | XBF_WRITE_FAIL);
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bp->b_flags |= XBF_WRITE | XBF_ASYNC;
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/*
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* we do all Io submission async. This means if we need to wait
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* for IO completion we need to take an extra reference so the
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* buffer is still valid on the other side.
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* We do all IO submission async. This means if we need
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* to wait for IO completion we need to take an extra
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* reference so the buffer is still valid on the other
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* side. We need to move the buffer onto the io_list
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* at this point so the caller can still access it.
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*/
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if (wait)
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bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_ASYNC | XBF_WRITE_FAIL);
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bp->b_flags |= XBF_WRITE | XBF_ASYNC;
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if (wait_list) {
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xfs_buf_hold(bp);
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else
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list_move_tail(&bp->b_list, wait_list);
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} else
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list_del_init(&bp->b_list);
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xfs_buf_submit(bp);
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@ -1849,8 +1860,7 @@ int
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xfs_buf_delwri_submit_nowait(
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struct list_head *buffer_list)
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{
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LIST_HEAD (io_list);
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return __xfs_buf_delwri_submit(buffer_list, &io_list, false);
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return xfs_buf_delwri_submit_buffers(buffer_list, NULL);
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}
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/*
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@ -1865,15 +1875,15 @@ int
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xfs_buf_delwri_submit(
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struct list_head *buffer_list)
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{
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LIST_HEAD (io_list);
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LIST_HEAD (wait_list);
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int error = 0, error2;
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struct xfs_buf *bp;
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__xfs_buf_delwri_submit(buffer_list, &io_list, true);
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xfs_buf_delwri_submit_buffers(buffer_list, &wait_list);
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/* Wait for IO to complete. */
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while (!list_empty(&io_list)) {
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bp = list_first_entry(&io_list, struct xfs_buf, b_list);
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while (!list_empty(&wait_list)) {
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bp = list_first_entry(&wait_list, struct xfs_buf, b_list);
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list_del_init(&bp->b_list);
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