linux-kernel - Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread.

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Date:   Wed, 5 Apr 2017 09:32:33 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device
 thread.

On Wed 05-04-17 09:19:27, Michal Hocko wrote:
> On Wed 05-04-17 14:33:50, NeilBrown wrote:
[...]
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 0ecb6461ed81..44b3506fd086 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -852,6 +852,7 @@ static int loop_prepare_queue(struct loop_device *lo)
> >  	if (IS_ERR(lo->worker_task))
> >  		return -ENOMEM;
> >  	set_user_nice(lo->worker_task, MIN_NICE);
> > +	lo->worker_task->flags |= PF_LESS_THROTTLE;
> >  	return 0;
> 
> As mentioned elsewhere, PF flags should be updated only on the current
> task otherwise there is potential rmw race. Is this safe? The code runs
> concurrently with the worker thread.

I believe you need something like this instead
---
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f347285c67ec..07b2a909e4fb 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -844,10 +844,16 @@ static void loop_unprepare_queue(struct loop_device *lo)
 	kthread_stop(lo->worker_task);
 }
 
+int loop_kthread_worker_fn(void *worker_ptr)
+{
+	current->flags |= PF_LESS_THROTTLE;
+	return kthread_worker_fn(worker_ptr);
+}
+
 static int loop_prepare_queue(struct loop_device *lo)
 {
 	kthread_init_worker(&lo->worker);
-	lo->worker_task = kthread_run(kthread_worker_fn,
+	lo->worker_task = kthread_run(loop_kthread_worker_fn,
 			&lo->worker, "loop%d", lo->lo_number);
 	if (IS_ERR(lo->worker_task))
 		return -ENOMEM;
-- 
Michal Hocko
SUSE Labs