Re: [PATCH 4/3 v2] xfs: async CIL flushes need pending pushes to be made stable

Matthew Wilcox
2022-03-08T13:52:27Z

On Tue, Mar 08, 2022 at 05:12:08PM +1100, Dave Chinner wrote:
> The fix is two-fold - first we should always set the
> cil->xc_push_commit_stable when xlog_cil_flush() is called,
> regardless of whether there is already a pending push or not.
>
> Second, if the CIL is empty, we should trigger an iclog flush to
> ensure that all the iclogs of the last checkpoint have actually been
> submitted to disk as that checkpoint may not have been run under
> stable completion constraints.
>
> Reported-by: Matthew Wilcox <willy@infradead.org>
> Fixes: 0020a190cf3e ("xfs: AIL needs asynchronous CIL forcing")
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> Version 2:
> - if the CIL is empty we need to push on the iclogs to ensure that
> the previous CIL flush did actually make it to stable storage.
> This closes a race condition where the AIL doesn't actually
> trigger the last CIL push and so the CIL is already empty by the
> time it tries to flush it to unpin pinned items.
>
> Note: this is against the XFS CIL scalability patchset. The
> XLOG_CIL_EMPTY check on a mainline kernel will be:
>
> if (list_empty(&log->l_cilp->xc_cil))
> xfs_log_force(log->l_mp, 0);

This is it. 100 runs of g/530 all completed in 2 or 3 seconds. Thanks!

Tested-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Just to be clear, my testing was done on b08968f196d4 (in Linus' tree),
plus my fs-folio patches, plus patches 1-3 from this series, plus this
(whitespace damaged):

+++ b/fs/xfs/xfs_log_cil.c
@@ -1243,18 +1243,27 @@ xlog_cil_push_now(
if (!async)
flush_workqueue(cil->xc_push_wq);

+ spin_lock(&cil->xc_push_lock);
+
+ /*
+ * If this is an async flush request, we always need to set the
+ * xc_push_commit_stable flag even if something else has already queued
+ * a push. The flush caller is asking for the CIL to be on stable
+ * storage when the next push completes, so regardless of who has queued
+ * the push, the flush requires stable semantics from it.
+ */
+ cil->xc_push_commit_stable = async;
+
/*
* If the CIL is empty or we've already pushed the sequence then
- * there's no work we need to do.
+ * there's no more work that we need to do.
*/
- spin_lock(&cil->xc_push_lock);
if (list_empty(&cil->xc_cil) || push_seq <= cil->xc_push_seq) {
spin_unlock(&cil->xc_push_lock);
return;
}

cil->xc_push_seq = push_seq;
- cil->xc_push_commit_stable = async;
queue_work(cil->xc_push_wq, &cil->xc_ctx->push_work);
spin_unlock(&cil->xc_push_lock);
}
@@ -1352,6 +1361,8 @@ xlog_cil_flush(

trace_xfs_log_force(log->l_mp, seq, _RET_IP_);
xlog_cil_push_now(log, seq, true);
+ if (list_empty(&log->l_cilp->xc_cil))
+ xfs_log_force(log->l_mp, 0);
}

/*