Re: [RFC PATCH 0/3] cgroup: fsio throttle controller

[Andrea Righi <righi.andrea@gmail.com>]

On Fri, Jan 18, 2019 at 02:46:53PM -0500, Josef Bacik wrote:
> On Fri, Jan 18, 2019 at 07:44:03PM +0100, Andrea Righi wrote:
> > On Fri, Jan 18, 2019 at 11:35:31AM -0500, Josef Bacik wrote:
> > > On Fri, Jan 18, 2019 at 11:31:24AM +0100, Andrea Righi wrote:
> > > > This is a redesign of my old cgroup-io-throttle controller:
> > > > https://lwn.net/Articles/330531/
> > > > 
> > > > I'm resuming this old patch to point out a problem that I think is still
> > > > not solved completely.
> > > > 
> > > > = Problem =
> > > > 
> > > > The io.max controller works really well at limiting synchronous I/O
> > > > (READs), but a lot of I/O requests are initiated outside the context of
> > > > the process that is ultimately responsible for its creation (e.g.,
> > > > WRITEs).
> > > > 
> > > > Throttling at the block layer in some cases is too late and we may end
> > > > up slowing down processes that are not responsible for the I/O that
> > > > is being processed at that level.
> > > 
> > > How so?  The writeback threads are per-cgroup and have the cgroup stuff set
> > > properly.  So if you dirty a bunch of pages, they are associated with your
> > > cgroup, and then writeback happens and it's done in the writeback thread
> > > associated with your cgroup and then that is throttled.  Then you are throttled
> > > at balance_dirty_pages() because the writeout is taking longer.
> > 
> > Right, writeback is per-cgroup and slowing down writeback affects only
> > that specific cgroup, but, there are cases where other processes from
> > other cgroups may require to wait on that writeback to complete before
> > doing I/O (for example an fsync() to a file shared among different
> > cgroups). In this case we may end up blocking cgroups that shouldn't be
> > blocked, that looks like a priority-inversion problem. This is the
> > problem that I'm trying to address.
> 
> Well this case is a misconfiguration, you shouldn't be sharing files between
> cgroups.  But even if you are, fsync() is synchronous, we should be getting the
> context from the process itself and thus should have its own rules applied.
> There's nothing we can do for outstanding IO, but that shouldn't be that much.
> That would need to be dealt with on a per-contoller basis.

OK, fair point. We shouldn't be sharing files between cgroups.

I'm still not sure if we can have similar issues with metadata I/O (that
may introduce latencies like the sync() scenario), I have to investigate
more and do more tests.

> 
> > 
> > > 
> > > I introduced the blk_cgroup_congested() stuff for paths that it's not easy to
> > > clearly tie IO to the thing generating the IO, such as readahead and such.  If
> > > you are running into this case that may be something worth using.  Course it
> > > only works for io.latency now but there's no reason you can't add support to it
> > > for io.max or whatever.
> > 
> > IIUC blk_cgroup_congested() is used in readahead I/O (and swap with
> > memcg), something like this: if the cgroup is already congested don't
> > generate extra I/O due to readahead. Am I right?
> 
> Yeah, but that's just how it's currently used, it can be used any which way we
> feel like.

I think it'd be very interesting to have the possibility to either
throttle I/O before writeback or during writeback. Right now we can only
throttle writeback. Maybe we can try to introduce some kind of dirty
page throttling controller using blk_cgroup_congested()... Opinions?

> 
> > 
> > > 
> > > > 
> > > > = Proposed solution =
> > > > 
> > > > The main idea of this controller is to split I/O measurement and I/O
> > > > throttling: I/O is measured at the block layer for READS, at page cache
> > > > (dirty pages) for WRITEs, and processes are limited while they're
> > > > generating I/O at the VFS level, based on the measured I/O.
> > > > 
> > > 
> > > This is what blk_cgroup_congested() is meant to accomplish, I would suggest
> > > looking into that route and simply changing the existing io controller you are
> > > using to take advantage of that so it will actually throttle things.  Then just
> > > sprinkle it around the areas where we indirectly generate IO.  Thanks,
> > 
> > Absolutely, I can probably use blk_cgroup_congested() as a method to
> > determine when a cgroup should be throttled (instead of doing my own
> > I/O measuring), but to prevent the "slow writeback slowing down other
> > cgroups" issue I still need to apply throttling when pages are dirtied
> > in page cache.
> 
> Again this is just a fuckup from a configuration stand point.  The argument
> could be made that sync() is probably broken here, but I think the right
> solution here is to just pass the cgroup context along with the writeback
> information and use that if it's set instead.  Thanks,

Alright, let's skip the root cgroup for now. I think the point here is
if we want to provide sync() isolation among cgroups or not.

According to the manpage:

       sync()  causes  all  pending  modifications  to filesystem metadata and cached file data to be
       written to the underlying filesystems.

And:
       According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but
       may  return  before  the actual writing is done.  However Linux waits for I/O completions, and
       thus sync() or syncfs() provide the same guarantees as fsync called on every file in the  sys‐
       tem or filesystem respectively.

Excluding the root cgroup, do you think a sync() issued inside a
specific cgroup should wait for I/O completions only for the writes that
have been generated by that cgroup?

Thanks,
-Andrea