Re: [dm-devel] [PATCH 1/2] swait: export the symbols __prepare_to_swait and __finish_swait

Re: [dm-devel] [PATCH 1/2] swait: export the symbols __prepare_to_swait and __finish_swait

[Christoph Hellwig]

Please run this past the swait authors.  It is supposed to be a simple
and self-contained API so I'd expect this patch to be seen critical.

You might be better off to just use the normal complex waitqueues if
you want to micro-optimize like this.

On Wed, Nov 22, 2017 at 09:35:36PM -0500, Mikulas Patocka wrote:
> In order to reduce locking overhead, I use the spinlock in
> swait_queue_head to protect not only the wait queue, but also the list of
> events. Consequently, I need to use unlocked functions __prepare_to_swait
> and __finish_swait. These functions are declared in the file
> include/linux/swait.h, but they are not exported, and so they are not
> useable from kernel modules.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  kernel/sched/swait.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> Index: linux-2.6/kernel/sched/swait.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/swait.c
> +++ linux-2.6/kernel/sched/swait.c
> @@ -73,6 +73,7 @@ void __prepare_to_swait(struct swait_que
>  	if (list_empty(&wait->task_list))
>  		list_add(&wait->task_list, &q->task_list);
>  }
> +EXPORT_SYMBOL(__prepare_to_swait);
>  
>  void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
>  {
> @@ -102,6 +103,7 @@ void __finish_swait(struct swait_queue_h
>  	if (!list_empty(&wait->task_list))
>  		list_del_init(&wait->task_list);
>  }
> +EXPORT_SYMBOL(__finish_swait);
>  
>  void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
>  {
> 
> --
> dm-devel mailing list
> dm-devel@redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
---end quoted text---

Re: [dm-devel] [PATCH 1/2] swait: export the symbols __prepare_to_swait and __finish_swait

[Mikulas Patocka]

On Thu, 23 Nov 2017, Christoph Hellwig wrote:

> Please run this past the swait authors.  It is supposed to be a simple
> and self-contained API so I'd expect this patch to be seen critical.

I already sent it to Peter Zijlstra and didn't get a response yet.

> You might be better off to just use the normal complex waitqueues if
> you want to micro-optimize like this.

If we wanted to micro-optimize, we should use the simpler wait queue 
variant.


If these functions are not supposed to be used by others, then
- why are they in in file swait.h?
- why does the implementation export swake_up_locked which assumes that 
someone else will lock the spinlock before calling it?

> On Wed, Nov 22, 2017 at 09:35:36PM -0500, Mikulas Patocka wrote:
> > In order to reduce locking overhead, I use the spinlock in
> > swait_queue_head to protect not only the wait queue, but also the list of
> > events. Consequently, I need to use unlocked functions __prepare_to_swait
> > and __finish_swait. These functions are declared in the file
> > include/linux/swait.h, but they are not exported, and so they are not
> > useable from kernel modules.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  kernel/sched/swait.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > Index: linux-2.6/kernel/sched/swait.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched/swait.c
> > +++ linux-2.6/kernel/sched/swait.c
> > @@ -73,6 +73,7 @@ void __prepare_to_swait(struct swait_que
> >  	if (list_empty(&wait->task_list))
> >  		list_add(&wait->task_list, &q->task_list);
> >  }
> > +EXPORT_SYMBOL(__prepare_to_swait);
> >  
> >  void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
> >  {
> > @@ -102,6 +103,7 @@ void __finish_swait(struct swait_queue_h
> >  	if (!list_empty(&wait->task_list))
> >  		list_del_init(&wait->task_list);
> >  }
> > +EXPORT_SYMBOL(__finish_swait);
> >  
> >  void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
> >  {
> > 
> > --
> > dm-devel mailing list
> > dm-devel@redhat.com
> > https://www.redhat.com/mailman/listinfo/dm-devel
> ---end quoted text---
> 

Re: [PATCH 1/2] swait: export the symbols __prepare_to_swait and __finish_swait

[Mike Snitzer]

On Thu, Nov 23 2017 at  5:27pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 23 Nov 2017, Christoph Hellwig wrote:
> 
> > Please run this past the swait authors.  It is supposed to be a simple
> > and self-contained API so I'd expect this patch to be seen critical.
> 
> I already sent it to Peter Zijlstra and didn't get a response yet.
> 
> > You might be better off to just use the normal complex waitqueues if
> > you want to micro-optimize like this.
> 
> If we wanted to micro-optimize, we should use the simpler wait queue 
> variant.
> 
> 
> If these functions are not supposed to be used by others, then
> - why are they in in file swait.h?
> - why does the implementation export swake_up_locked which assumes that 
> someone else will lock the spinlock before calling it?

Hi,

I'd like to get this patch upstream.  I'm happy to send it to Linus via
linux-dm.git but I wanted to check with others who might care more
deeply about swait interfaces to get their Ack (or otherwise):
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.18&id=4a2ec3f321f83db09da4824025420586c9ef1612

Here is Mikulas' DM driver code that makes use of __prepare_to_swait()
and __finish_swait():

static int writecache_endio_thread(void *data)
{
        struct dm_writecache *wc = data;

        while (1) {
                DECLARE_SWAITQUEUE(wait);
                struct list_head list;

                raw_spin_lock_irq(&wc->endio_thread_wait.lock);
continue_locked:
                if (!list_empty(&wc->endio_list))
                        goto pop_from_list;
                set_current_state(TASK_INTERRUPTIBLE);
                __prepare_to_swait(&wc->endio_thread_wait, &wait);
                raw_spin_unlock_irq(&wc->endio_thread_wait.lock);

                if (unlikely(kthread_should_stop())) {
                        finish_swait(&wc->endio_thread_wait, &wait);
                        break;
                }

                schedule();

                raw_spin_lock_irq(&wc->endio_thread_wait.lock);
                __finish_swait(&wc->endio_thread_wait, &wait);
                goto continue_locked;

pop_from_list:
                list = wc->endio_list;
                list.next->prev = list.prev->next = &list;
                INIT_LIST_HEAD(&wc->endio_list);
                raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
                ...