Re: [patch v3]DM: dm-insitu-comp: a compressed DM target for SSD

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Mar 18 2014 at  3:41am -0400,
Shaohua Li <shli@kernel.org> wrote:

> On Mon, Mar 17, 2014 at 04:00:40PM -0400, Mike Snitzer wrote:
> > 
> > I folded your changes in, and then committed a patch ontop that cleans
> > some code up.  But added 2 FIXMEs that still speak to pretty fundamental
> > problems with the architecture of the dm-insitu-comp target, see:
> > https://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=for-3.15-insitu-comp&id=8565ab6b04837591d03c94851c2f9f9162ce12f4
> > 
> > Unfortunately the single insitu_comp_wq workqueue that all insitu-comp
> > targets are to share isn't a workable solution.  Each target needs to
> > have resource isolation from other targets (imagine insitu-comp used for
> > multiple SSDs).  This is important for suspend too because you'll need
> > to flush/stop the workqueue.
> 
> Is this just because of suspend? I didn't see fundamental reason why the
> workqueue can't be shared even for several targets.

I'm not seeing how you are guaranteeing that all queued work is
completed during suspend.  insitu_comp_queue_req() just calls
queue_work_on().

BTW, queue_work_on()'s comment above its implementation says:
"We queue the work to a specific CPU, the caller must ensure it can't go
away." -- you're not able to insure a cpu isn't hotplugged so... but I
also see you've used it in your raid5 perf improvement changes so you
obviously have experience with using this interface.

> > You introduced a state machine for tracking suspending, suspended,
> > resumed.  This really isn't necessary.  During suspend you need to
> > flush_workqueue().  On resume you shouldn't need to do anything special.
> > 
> > As I noted in the commit, the thin and cache targets can serve as
> > references for how you can manage the workqueue across suspend/resume
> > and the lifetime of these workqueues relative to .ctr and .dtr.
> 
> As far as I checking the code, .postsuspend is called after all requests are
> finished. This already guarantees no pending requests running in insitu-comp
> workqueue.

I could easily be missing something obvious, but I don't see where that
guarantee is implemented.

> Doing a workqueue flush isn't required. The writeback thread is
> running in background and waiting for requests completion can't guarantee the
> thread isn't running, so we must make sure it is safely parked.

Sure, but you don't need a state machine to do that.  The DM core takes
care of calling these hooks, so you just need to stop the writeback
thread during suspend and (re)start/kick it on resume (preresume).