Messages in this thread | | | Date | Wed, 8 Jun 2011 07:51:28 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 02/15] writeback: update dirtied_when for synced inode to prevent livelock |
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On Wed, Jun 08, 2011 at 07:02:45AM +0800, Andrew Morton wrote: > On Wed, 08 Jun 2011 05:32:38 +0800 > Wu Fengguang <fengguang.wu@intel.com> wrote: > > > Explicitly update .dirtied_when on synced inodes, so that they are no > > longer considered for writeback in the next round. > > It sounds like this somewhat answers my questions for [1/15]. > > But I'm not seeing a description of exactly what caused the livelock.
The exact livelock condition is, during sync(1):
(1) no new inodes are dirtied (2) an inode being actively dirtied
On (2), the inode will be tagged and synced with .nr_to_write=LONG_MAX. When finished, it will be redirty_tail()ed because it's still dirty and (.nr_to_write > 0). redirty_tail() won't update its ->dirtied_when on condition (1). The sync work will then revisit it on the next queue_io() and find it eligible again because its old ->dirtied_when predates the sync work start time.
I'll add the above to the changelog.
> > We'll do more aggressive "keep writeback as long as we wrote something" > > logic in wb_writeback(). The "use LONG_MAX .nr_to_write" trick in commit > > b9543dac5bbc ("writeback: avoid livelocking WB_SYNC_ALL writeback") will > > no longer be enough to stop sync livelock. > > > > It can prevent both of the following livelock schemes: > > > > - while true; do echo data >> f; done > > - while true; do touch f; done > > You're kidding. This livelocks sync(1)? When did we break this?
There are no reported real cases for "touch f" style livelock. It's merely a possibility in theory and the more concurrent meta data dirties, the more likelihood it will happen.
> Why is this? Because the inode keeps on getting rotated to head-of-list?
Yes, when the inode is always redirty_tail()ed without updating its ->dirtied_when.
Thanks, Fengguang
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