Re: [PATCH 3/5] writeback: stop background/kupdate works from livelocking other works

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 10 Nov 2010 00:56:32 +0100
Jan Kara <jack@suse.cz> wrote:

> On Tue 09-11-10 15:00:06, Andrew Morton wrote:
> > On Tue, 9 Nov 2010 23:28:27 +0100
> > Jan Kara <jack@suse.cz> wrote:
> > >   New description which should address above questions:
> > > Background writeback is easily livelockable in a loop in wb_writeback() by
> > > a process continuously re-dirtying pages (or continuously appending to a
> > > file). This is in fact intended as the target of background writeback is to
> > > write dirty pages it can find as long as we are over
> > > dirty_background_threshold.
> > 
> > Well.  The objective of the kupdate function is utterly different.
> > 
> > > But the above behavior gets inconvenient at times because no other work
> > > queued in the flusher thread's queue gets processed. In particular,
> > > since e.g. sync(1) relies on flusher thread to do all the IO for it,
> > 
> > That's fixable by doing the work synchronously within sync_inodes_sb(),
> > rather than twiddling thumbs wasting a thread resource while waiting
> > for kernel threads to do it.  As an added bonus, this even makes cpu
> > time accounting more accurate ;)
> > 
> > Please remind me why we decided to hand the sync_inodes_sb() work off
> > to other threads?
>   Because when sync(1) does IO on it's own, it competes for the device with
> the flusher thread running in parallel thus resulting in more seeks.

Skeptical.  Has that effect been demonstrated?  Has it been shown to be
a significant problem?  A worse problem than livelocking the machine? ;)

If this _is_ a problem then it's also a problem for fsync/msync.  But
see below.

> > > sync(1) can hang forever waiting for flusher thread to do the work.
> > > 
> > > Generally, when a flusher thread has some work queued, someone submitted
> > > the work to achieve a goal more specific than what background writeback
> > > does. Moreover by working on the specific work, we also reduce amount of
> > > dirty pages which is exactly the target of background writeout. So it makes
> > > sense to give specific work a priority over a generic page cleaning.
> > > 
> > > Thus we interrupt background writeback if there is some other work to do. We
> > > return to the background writeback after completing all the queued work.
> > > 
> ...
> > > > So...  what prevents higher priority works (eg, sync(1)) from
> > > > livelocking or seriously retarding background or kudate writeout?
> > >   If other work than background or kupdate writeout livelocks, it's a bug
> > > which should be fixed (either by setting sensible nr_to_write or by tagging
> > > like we do it for WB_SYNC_ALL writeback). Of course, higher priority work
> > > can be running when background or kupdate writeout would need to run as
> > > well. But the idea here is that the purpose of background/kupdate types of
> > > writeout is to get rid of dirty data and any type of writeout does this so
> > > working on it we also work on background/kupdate writeout only possibly
> > > less efficiently.
> > 
> > The kupdate function is a data-integrity/quality-of-service sort of
> > thing.
> > 
> > And what I'm asking is whether this change enables scenarios in which
> > these threads can be kept so busy that the kupdate function gets
> > interrupted so frequently that we can have dirty memory not being
> > written back for arbitrarily long periods of time?
>   So let me compare:
> What kupdate writeback does:
>   queue inodes older than dirty_expire_centisecs
>   while some inode in the queue
>     write MAX_WRITEBACK_PAGES from each inode queued
>     break if nr_to_write <= 0
> 
> What any other WB_SYNC_NONE writeback (let me call it "normal WB_SYNC_NONE
> writeback") does:
>   queue all dirty inodes 
>   while some inode in the queue
>     write MAX_WRITEBACK_PAGES from each inode queued
>     break if nr_to_write <= 0
> 
> 
> There only one kind of WB_SYNC_ALL writeback - the one which writes
> everything.

fsync() uses WB_SYNC_ALL and it doesn't write "everything".

> So after WB_SYNC_ALL writeback (provided all livelocks are fixed ;)
> obviously no old data should be unwritten in memory. Normal WB_SYNC_NONE
> writeback differs from a kupdate one *only* in the fact that we queue all
> inodes instead of only the old ones.

whoa.

That's only true for sync(), or sync_filesystem().  It isn't true for,
say, fsync() and msync().  Now when someone comes along and changes
fsync/msync to use flusher threads ("resulting in less seeks") then we
could get into a situation where fsync-serving flusher threads never
serve kupdate?

> We start writing old inodes first and

OT, but: your faith in those time-ordered inode lists is touching ;)
Put a debug function in there which checks that the lists _are_
time-ordered, and call that function from every site in the kernel
which modifies the lists.   I bet there are still gremlins.

> go inode by inode writing MAX_WRITEBACK_PAGES from each. Now because the
> queue can be longer for normal WB_SYNC_NONE writeback, it can take longer
> before we return to the old inodes. So if normal writeback interrupts
> kupdate one, it can take longer before all data of old inodes get to disk.
> But we always get the old data to disk - essentially at the same time at
> which kupdate writeback would get them to disk if dirty_expire_centisecs
> was 0.
> 
> Is this enough? Do you want any of this in the changelog?
> 
> Thanks for the inquiry btw. It made me cleanup my thoughts on the subject ;)
> 

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