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xfs: prevent CIL push holdoff in log recovery
generic/530 on a machine with enough ram and a non-preemptible kernel can run the AGI processing phase of log recovery enitrely out of cache. This means it never blocks on locks, never waits for IO and runs entirely through the unlinked lists until it either completes or blocks and hangs because it has run out of log space. It runs out of log space because the background CIL push is scheduled but never runs. queue_work() queues the CIL work on the current CPU that is busy, and the workqueue code will not run it on any other CPU. Hence if the unlinked list processing never yields the CPU voluntarily, the push work is delayed indefinitely. This results in the CIL aggregating changes until all the log space is consumed. When the log recoveyr processing evenutally blocks, the CIL flushes but because the last iclog isn't submitted for IO because it isn't full, the CIL flush never completes and nothing ever moves the log head forwards, or indeed inserts anything into the tail of the log, and hence nothing is able to get the log moving again and recovery hangs. There are several problems here, but the two obvious ones from the trace are that: a) log recovery does not yield the CPU for over 4 seconds, b) binding CIL pushes to a single CPU is a really bad idea. This patch addresses just these two aspects of the problem, and are suitable for backporting to work around any issues in older kernels. The more fundamental problem of preventing the CIL from consuming more than 50% of the log without committing will take more invasive and complex work, so will be done as followup work. Signed-off-by:Dave Chinner <dchinner@redhat.com> Reviewed-by: