xfs: Fix CIL throttle hang when CIL space used going backwards
A hang with tasks stuck on the CIL hard throttle was reported and largely diagnosed by Donald Buczek, who discovered that it was a result of the CIL context space usage decrementing in committed transactions once the hard throttle limit had been hit and processes were already blocked. This resulted in the CIL push not waking up those waiters because the CIL context was no longer over the hard throttle limit. The surprising aspect of this was the CIL space usage going backwards regularly enough to trigger this situation. Assumptions had been made in design that the relogging process would only increase the size of the objects in the CIL, and so that space would only increase. This change and commit message fixes the issue and documents the result of an audit of the triggers that can cause the CIL space to go backwards, how large the backwards steps tend to be, the frequency in which they occur, and what the impact on the CIL accounting code is. Even though the CIL ctx->space_used can go backwards, it will only do so if the log item is already logged to the CIL and contains a space reservation for it's entire logged state. This is tracked by the shadow buffer state on the log item. If the item is not previously logged in the CIL it has no shadow buffer nor log vector, and hence the entire size of the logged item copied to the log vector is accounted to the CIL space usage. i.e. it will always go up in this case. If the item has a log vector (i.e. already in the CIL) and the size decreases, then the existing log vector will be overwritten and the space usage will go down. This is the only condition where the space usage reduces, and it can only occur when an item is already tracked in the CIL. Hence we are safe from CIL space usage underruns as a result of log items decreasing in size when they are relogged. Typically this reduction in CIL usage occurs from metadata blocks being free, such as when a btree block merge occurs or a directory enter/xattr entry is removed and the da-tree is reduced in size. This generally results in a reduction in size of around a single block in the CIL, but also tends to increase the number of log vectors because the parent and sibling nodes in the tree needs to be updated when a btree block is removed. If a multi-level merge occurs, then we see reduction in size of 2+ blocks, but again the log vector count goes up. The other vector is inode fork size changes, which only log the current size of the fork and ignore the previously logged size when the fork is relogged. Hence if we are removing items from the inode fork (dir/xattr removal in shortform, extent record removal in extent form, etc) the relogged size of the inode for can decrease. No other log items can decrease in size either because they are a fixed size (e.g. dquots) or they cannot be relogged (e.g. relogging an intent actually creates a new intent log item and doesn't relog the old item at all.) Hence the only two vectors for CIL context size reduction are relogging inode forks and marking buffers active in the CIL as stale. Long story short: the majority of the code does the right thing and handles the reduction in log item size correctly, and only the CIL hard throttle implementation is problematic and needs fixing. This patch makes that fix, as well as adds comments in the log item code that result in items shrinking in size when they are relogged as a clear reminder that this can and does happen frequently. The throttle fix is based upon the change Donald proposed, though it goes further to ensure that once the throttle is activated, it captures all tasks until the CIL push issues a wakeup, regardless of whether the CIL space used has gone back under the throttle threshold. This ensures that we prevent tasks reducing the CIL slightly under the throttle threshold and then making more changes that push it well over the throttle limit. This is acheived by checking if the throttle wait queue is already active as a condition of throttling. Hence once we start throttling, we continue to apply the throttle until the CIL context push wakes everything on the wait queue. We can use waitqueue_active() for the waitqueue manipulations and checks as they are all done under the ctx->xc_push_lock. Hence the waitqueue has external serialisation and we can safely peek inside the wait queue without holding the internal waitqueue locks. Many thanks to Donald for his diagnostic and analysis work to isolate the cause of this hang. Reported-and-tested-by:Donald Buczek <buczek@molgen.mpg.de> Signed-off-by:
Dave Chinner <dchinner@redhat.com> Reviewed-by:
Brian Foster <bfoster@redhat.com> Reviewed-by:
Chandan Babu R <chandanrlinux@gmail.com> Reviewed-by:
Darrick J. Wong <djwong@kernel.org> Reviewed-by:
Allison Henderson <allison.henderson@oracle.com> Signed-off-by:
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