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

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Message-ID: <20140319014557.GA1631@kernel.org>
Date:	Wed, 19 Mar 2014 09:45:57 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	axboe@...nel.dk, dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	agk@...hat.com
Subject: Re: [patch v3]DM: dm-insitu-comp: a compressed DM target for SSD

On Tue, Mar 18, 2014 at 05:28:43PM -0400, Mike Snitzer wrote:
> On Tue, Mar 18 2014 at  3:41am -0400,
> Shaohua Li <shli@...nel.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.

Good point, I did miss this. the raid5 case hold a lock, while this case
doesn't. A fix is attached below.

> > > 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.

Alright, so this is the divergence. dm_suspend() calls dm_wait_for_completion()
and then dm_table_postsuspend_targets(). As far as I understand,
dm_wait_for_completion() will wait all pending requests to finish. The comments
declaim this too. Am I missing anything?

Basically the two kinds of IO. IO requests from upper layer, which
dm_wait_for_completion() will guarantee they are finished. Metadata IO
requests, which .postsuspend should make sure they are finished.
 
> > 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).

Yep, I need wait the writeback thread finish all pending metadata IO, the state
machine works well here.


---
 drivers/md/dm-insitu-comp.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: linux/drivers/md/dm-insitu-comp.c
===================================================================
--- linux.orig/drivers/md/dm-insitu-comp.c	2014-03-17 17:40:01.106660303 +0800
+++ linux/drivers/md/dm-insitu-comp.c	2014-03-19 08:49:12.314627050 +0800
@@ -704,14 +704,19 @@ static void insitu_comp_queue_req(struct
 				  struct insitu_comp_req *req)
 {
 	unsigned long flags;
-	struct insitu_comp_io_worker *worker =
-		&insitu_comp_io_workers[req->cpu];
+	struct insitu_comp_io_worker *worker;
+
+	preempt_disable();
+	if (!cpu_online(req->cpu))
+		req->cpu = cpumask_any(cpu_online_mask);
+	worker = &insitu_comp_io_workers[req->cpu];
 
 	spin_lock_irqsave(&worker->lock, flags);
 	list_add_tail(&req->sibling, &worker->pending);
 	spin_unlock_irqrestore(&worker->lock, flags);
 
 	queue_work_on(req->cpu, insitu_comp_wq, &worker->work);
+	preempt_enable();
 }
 
 static void insitu_comp_queue_req_list(struct insitu_comp_info *info,
--
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