Re: [PATCH] mmc: block: handle complete_work on separate workqueue

Re: [PATCH] mmc: block: handle complete_work on separate workqueue

[Ulf Hansson]

+ Adrian, Linus

On Tue, 5 Feb 2019 at 17:53, Zachary Hays <zhays@lexmark.com> wrote:
>
> The kblockd workqueue is created with the WQ_MEM_RECLAIM flag set.
> This generates a rescuer thread for that queue that will trigger when
> the CPU is under heavy load and collect the uncompleted work.
>
> In the case of mmc, this creates the possibility of a deadlock as
> other blk-mq work is also run on the same queue. For example:
>
> - worker 0 claims the mmc host
> - worker 1 attempts to claim the host
> - worker 0 schedules complete_work to release the host
> - rescuer thread is triggered after time-out and collects the dangling
>   work
> - rescuer thread attempts to complete the work in order starting with
>   claim host
> - the task to release host is now blocked by a task to claim it and
>   will never be called
>

Adrian, I need your help to understand more of this. The above
description doesn't add up to me.

In principle, already when "worker 1 attempts to claim the host" as
described above, it should succeed and should *not* need to wait for
the host to be released. Right?

The hole point with the commit 6c0cedd1ef95 ("mmc: core: Introduce
host claiming by context"), was to allow the mmc host to be
re-claimable for blk I/O requests, no matter from what worker/thread
the claim/release is done from.

Is it not working as expected you think? What am I missing here?

> The above results in multiple hung tasks that lead to failures to boot.

Of course, during boot there is also a card detect work running in
parallel with blk I/O requests. This work is being punted to the
"system_freezable_wq" and it claims/releases the host as well.

Perhaps, that could have something to do with the problem....

>
> Handling complete_work on a separate workqueue avoids this by keeping
> the work completion tasks separate from the other blk-mq work. This
> allows the host to be released without getting blocked by other tasks
> attempting to claim the host.
>
> Signed-off-by: Zachary Hays <zhays@lexmark.com>
> ---
>  drivers/mmc/core/block.c | 10 +++++++++-
>  include/linux/mmc/card.h |  1 +
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
> index aef1185f383d..14f3fdb8c6bb 100644
> --- a/drivers/mmc/core/block.c
> +++ b/drivers/mmc/core/block.c
> @@ -2112,7 +2112,7 @@ static void mmc_blk_mq_req_done(struct mmc_request *mrq)
>                 if (waiting)
>                         wake_up(&mq->wait);
>                 else
> -                       kblockd_schedule_work(&mq->complete_work);
> +                       queue_work(mq->card->complete_wq, &mq->complete_work);
>
>                 return;
>         }
> @@ -2924,6 +2924,13 @@ static int mmc_blk_probe(struct mmc_card *card)
>
>         mmc_fixup_device(card, mmc_blk_fixups);
>
> +       card->complete_wq = alloc_workqueue("mmc_complete",
> +                                       WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
> +       if (unlikely(!card->complete_wq)) {
> +               pr_err("Failed to create mmc completion workqueue");
> +               return -ENOMEM;
> +       }
> +
>         md = mmc_blk_alloc(card);
>         if (IS_ERR(md))
>                 return PTR_ERR(md);
> @@ -2987,6 +2994,7 @@ static void mmc_blk_remove(struct mmc_card *card)
>         pm_runtime_put_noidle(&card->dev);
>         mmc_blk_remove_req(md);
>         dev_set_drvdata(&card->dev, NULL);
> +       destroy_workqueue(card->complete_wq);
>  }
>
>  static int _mmc_blk_suspend(struct mmc_card *card)
> diff --git a/include/linux/mmc/card.h b/include/linux/mmc/card.h
> index de7377815b6b..8ef330027b13 100644
> --- a/include/linux/mmc/card.h
> +++ b/include/linux/mmc/card.h
> @@ -308,6 +308,7 @@ struct mmc_card {
>         unsigned int    nr_parts;
>
>         unsigned int            bouncesz;       /* Bounce buffer size */
> +       struct workqueue_struct *complete_wq;   /* Private workqueue */
>  };
>
>  static inline bool mmc_large_sector(struct mmc_card *card)
> --
> 2.7.4
>

So this change seems to solve the problem, which is really great.
However, I think we really need to understand what goes wrong, before
we apply this as fix.

Unfortunate, I am also in busy period, so I need a couple of more days
before I can help out running tests.

Kind regards
Uffe

Re: [PATCH] mmc: block: handle complete_work on separate workqueue

[Adrian Hunter]

On 6/02/19 2:00 PM, Ulf Hansson wrote:
> + Adrian, Linus
> 
> On Tue, 5 Feb 2019 at 17:53, Zachary Hays <zhays@lexmark.com> wrote:
>>
>> The kblockd workqueue is created with the WQ_MEM_RECLAIM flag set.
>> This generates a rescuer thread for that queue that will trigger when
>> the CPU is under heavy load and collect the uncompleted work.
>>
>> In the case of mmc, this creates the possibility of a deadlock as
>> other blk-mq work is also run on the same queue. For example:
>>
>> - worker 0 claims the mmc host
>> - worker 1 attempts to claim the host
>> - worker 0 schedules complete_work to release the host
>> - rescuer thread is triggered after time-out and collects the dangling
>>   work
>> - rescuer thread attempts to complete the work in order starting with
>>   claim host
>> - the task to release host is now blocked by a task to claim it and
>>   will never be called
>>
> 
> Adrian, I need your help to understand more of this. The above
> description doesn't add up to me.
> 
> In principle, already when "worker 1 attempts to claim the host" as
> described above, it should succeed and should *not* need to wait for
> the host to be released. Right?

If it is the same queue, then yes.  Although in that case there is only 1
work for the hw queue so there cannot be another worker.  There could be
another attempt to send a request directly, but that will not block - if the
host controller is busy, BLK_STS_RESOURCE will be returned from ->queue_rq().

> 
> The hole point with the commit 6c0cedd1ef95 ("mmc: core: Introduce
> host claiming by context"), was to allow the mmc host to be
> re-claimable for blk I/O requests, no matter from what worker/thread
> the claim/release is done from.
> 
> Is it not working as expected you think? What am I missing here?

I assumed we were talking about a situation where there are multiple
internal eMMC partitions each with their own disk and queue.  In that case,
a queue waits if there is another queue that is using the eMMC.

We should clarify whether that is the scenario we are looking at.  Zachary?

> 
>> The above results in multiple hung tasks that lead to failures to boot.
> 
> Of course, during boot there is also a card detect work running in
> parallel with blk I/O requests. This work is being punted to the
> "system_freezable_wq" and it claims/releases the host as well.
> 
> Perhaps, that could have something to do with the problem....
> 
>>
>> Handling complete_work on a separate workqueue avoids this by keeping
>> the work completion tasks separate from the other blk-mq work. This
>> allows the host to be released without getting blocked by other tasks
>> attempting to claim the host.
>>
>> Signed-off-by: Zachary Hays <zhays@lexmark.com>
>> ---
>>  drivers/mmc/core/block.c | 10 +++++++++-
>>  include/linux/mmc/card.h |  1 +
>>  2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
>> index aef1185f383d..14f3fdb8c6bb 100644
>> --- a/drivers/mmc/core/block.c
>> +++ b/drivers/mmc/core/block.c
>> @@ -2112,7 +2112,7 @@ static void mmc_blk_mq_req_done(struct mmc_request *mrq)
>>                 if (waiting)
>>                         wake_up(&mq->wait);
>>                 else
>> -                       kblockd_schedule_work(&mq->complete_work);
>> +                       queue_work(mq->card->complete_wq, &mq->complete_work);
>>
>>                 return;
>>         }
>> @@ -2924,6 +2924,13 @@ static int mmc_blk_probe(struct mmc_card *card)
>>
>>         mmc_fixup_device(card, mmc_blk_fixups);
>>
>> +       card->complete_wq = alloc_workqueue("mmc_complete",
>> +                                       WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
>> +       if (unlikely(!card->complete_wq)) {
>> +               pr_err("Failed to create mmc completion workqueue");
>> +               return -ENOMEM;
>> +       }
>> +
>>         md = mmc_blk_alloc(card);
>>         if (IS_ERR(md))
>>                 return PTR_ERR(md);
>> @@ -2987,6 +2994,7 @@ static void mmc_blk_remove(struct mmc_card *card)
>>         pm_runtime_put_noidle(&card->dev);
>>         mmc_blk_remove_req(md);
>>         dev_set_drvdata(&card->dev, NULL);
>> +       destroy_workqueue(card->complete_wq);
>>  }
>>
>>  static int _mmc_blk_suspend(struct mmc_card *card)
>> diff --git a/include/linux/mmc/card.h b/include/linux/mmc/card.h
>> index de7377815b6b..8ef330027b13 100644
>> --- a/include/linux/mmc/card.h
>> +++ b/include/linux/mmc/card.h
>> @@ -308,6 +308,7 @@ struct mmc_card {
>>         unsigned int    nr_parts;
>>
>>         unsigned int            bouncesz;       /* Bounce buffer size */
>> +       struct workqueue_struct *complete_wq;   /* Private workqueue */
>>  };
>>
>>  static inline bool mmc_large_sector(struct mmc_card *card)
>> --
>> 2.7.4
>>
> 
> So this change seems to solve the problem, which is really great.
> However, I think we really need to understand what goes wrong, before
> we apply this as fix.
> 
> Unfortunate, I am also in busy period, so I need a couple of more days
> before I can help out running tests.
> 
> Kind regards
> Uffe
> 

Re: [PATCH] mmc: block: handle complete_work on separate workqueue

[Ulf Hansson]

On Wed, 6 Feb 2019 at 13:43, Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 6/02/19 2:00 PM, Ulf Hansson wrote:
> > + Adrian, Linus
> >
> > On Tue, 5 Feb 2019 at 17:53, Zachary Hays <zhays@lexmark.com> wrote:
> >>
> >> The kblockd workqueue is created with the WQ_MEM_RECLAIM flag set.
> >> This generates a rescuer thread for that queue that will trigger when
> >> the CPU is under heavy load and collect the uncompleted work.
> >>
> >> In the case of mmc, this creates the possibility of a deadlock as
> >> other blk-mq work is also run on the same queue. For example:
> >>
> >> - worker 0 claims the mmc host
> >> - worker 1 attempts to claim the host
> >> - worker 0 schedules complete_work to release the host
> >> - rescuer thread is triggered after time-out and collects the dangling
> >>   work
> >> - rescuer thread attempts to complete the work in order starting with
> >>   claim host
> >> - the task to release host is now blocked by a task to claim it and
> >>   will never be called
> >>
> >
> > Adrian, I need your help to understand more of this. The above
> > description doesn't add up to me.
> >
> > In principle, already when "worker 1 attempts to claim the host" as
> > described above, it should succeed and should *not* need to wait for
> > the host to be released. Right?
>
> If it is the same queue, then yes.  Although in that case there is only 1
> work for the hw queue so there cannot be another worker.  There could be
> another attempt to send a request directly, but that will not block - if the
> host controller is busy, BLK_STS_RESOURCE will be returned from ->queue_rq().
>

Right.

> >
> > The hole point with the commit 6c0cedd1ef95 ("mmc: core: Introduce
> > host claiming by context"), was to allow the mmc host to be
> > re-claimable for blk I/O requests, no matter from what worker/thread
> > the claim/release is done from.
> >
> > Is it not working as expected you think? What am I missing here?
>
> I assumed we were talking about a situation where there are multiple
> internal eMMC partitions each with their own disk and queue.  In that case,
> a queue waits if there is another queue that is using the eMMC.

Of course! I totally forgot about this case, that is most certainly
what must be happening!

>
> We should clarify whether that is the scenario we are looking at.  Zachary?

Yes, please.

Assuming this is the case, I would also prefer an updated changelog
that describe this scenario.

Adrian, thanks a lot for you help!

[...]

Kind regards
Uffe

RE: [PATCH] mmc: block: handle complete_work on separate workqueue

[Zak Hays]

> -----Original Message-----
> From: Ulf Hansson <ulf.hansson@linaro.org>
> Sent: Wednesday, February 6, 2019 9:08 AM
> To: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Zak Hays <zak.hays@lexmark.com>; Christoph Hellwig <hch@lst.de>;
> linux-mmc@vger.kernel.org; linux-block <linux-block@vger.kernel.org>;
> Linus Walleij <linus.walleij@linaro.org>
> Subject: Re: [PATCH] mmc: block: handle complete_work on separate
> workqueue
> 
> On Wed, 6 Feb 2019 at 13:43, Adrian Hunter <adrian.hunter@intel.com>
> wrote:
> >
> > On 6/02/19 2:00 PM, Ulf Hansson wrote:
> > > + Adrian, Linus
> > >
> > > On Tue, 5 Feb 2019 at 17:53, Zachary Hays <zhays@lexmark.com> wrote:
> > >>
> > >> The kblockd workqueue is created with the WQ_MEM_RECLAIM flag
> set.
> > >> This generates a rescuer thread for that queue that will trigger when
> > >> the CPU is under heavy load and collect the uncompleted work.
> > >>
> > >> In the case of mmc, this creates the possibility of a deadlock as
> > >> other blk-mq work is also run on the same queue. For example:
> > >>
> > >> - worker 0 claims the mmc host
> > >> - worker 1 attempts to claim the host
> > >> - worker 0 schedules complete_work to release the host
> > >> - rescuer thread is triggered after time-out and collects the dangling
> > >>   work
> > >> - rescuer thread attempts to complete the work in order starting with
> > >>   claim host
> > >> - the task to release host is now blocked by a task to claim it and
> > >>   will never be called
> > >>
> > >
> > > Adrian, I need your help to understand more of this. The above
> > > description doesn't add up to me.
> > >
> > > In principle, already when "worker 1 attempts to claim the host" as
> > > described above, it should succeed and should *not* need to wait for
> > > the host to be released. Right?
> >
> > If it is the same queue, then yes.  Although in that case there is only 1
> > work for the hw queue so there cannot be another worker.  There could be
> > another attempt to send a request directly, but that will not block - if the
> > host controller is busy, BLK_STS_RESOURCE will be returned from -
> >queue_rq().
> >
> 
> Right.
> 
> > >
> > > The hole point with the commit 6c0cedd1ef95 ("mmc: core: Introduce
> > > host claiming by context"), was to allow the mmc host to be
> > > re-claimable for blk I/O requests, no matter from what worker/thread
> > > the claim/release is done from.
> > >
> > > Is it not working as expected you think? What am I missing here?
> >
> > I assumed we were talking about a situation where there are multiple
> > internal eMMC partitions each with their own disk and queue.  In that case,
> > a queue waits if there is another queue that is using the eMMC.
> 
> Of course! I totally forgot about this case, that is most certainly
> what must be happening!
> 
> >
> > We should clarify whether that is the scenario we are looking at.  Zachary?
> 
> Yes, please.

Yes, this is the case. There are multiple partitions on the eMMC.

> 
> Assuming this is the case, I would also prefer an updated changelog
> that describe this scenario.

I will update the commit message to clarify this and resubmit.

> Adrian, thanks a lot for you help!
> 
> [...]
> 
> Kind regards
> Uffe

Re: [PATCH v2] mmc: block: handle complete_work on separate workqueue

[Ulf Hansson]

On Thu, 7 Feb 2019 at 16:55, Zachary Hays <zhays@lexmark.com> wrote:
>
> The kblockd workqueue is created with the WQ_MEM_RECLAIM flag set.
> This generates a rescuer thread for that queue that will trigger when
> the CPU is under heavy load and collect the uncompleted work.
>
> In the case of mmc, this creates the possibility of a deadlock when
> there are multiple partitions on the device as other blk-mq work is
> also run on the same queue. For example:
>
> - worker 0 claims the mmc host to work on partition 1
> - worker 1 attempts to claim the host for partition 2 but has to wait
>   for worker 0 to finish
> - worker 0 schedules complete_work to release the host
> - rescuer thread is triggered after time-out and collects the dangling
>   work
> - rescuer thread attempts to complete the work in order starting with
>   claim host
> - the task to release host is now blocked by a task to claim it and
>   will never be called
>
> The above results in multiple hung tasks that lead to failures to
> mount partitions.
>
> Handling complete_work on a separate workqueue avoids this by keeping
> the work completion tasks separate from the other blk-mq work. This
> allows the host to be released without getting blocked by other tasks
> attempting to claim the host.
>
> Signed-off-by: Zachary Hays <zhays@lexmark.com>

Applied for fixes and by adding a fixes+stable tag.

Thanks Zachary for fixing this non-trivial problem! And thanks also to
Adrian/Christoph for your valuable input to this (feel free to reply
with your reviewed-by tag).

Kind regards
Uffe


> ---
>  drivers/mmc/core/block.c | 10 +++++++++-
>  include/linux/mmc/card.h |  1 +
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
> index aef1185f383d..14f3fdb8c6bb 100644
> --- a/drivers/mmc/core/block.c
> +++ b/drivers/mmc/core/block.c
> @@ -2112,7 +2112,7 @@ static void mmc_blk_mq_req_done(struct mmc_request *mrq)
>                 if (waiting)
>                         wake_up(&mq->wait);
>                 else
> -                       kblockd_schedule_work(&mq->complete_work);
> +                       queue_work(mq->card->complete_wq, &mq->complete_work);
>
>                 return;
>         }
> @@ -2924,6 +2924,13 @@ static int mmc_blk_probe(struct mmc_card *card)
>
>         mmc_fixup_device(card, mmc_blk_fixups);
>
> +       card->complete_wq = alloc_workqueue("mmc_complete",
> +                                       WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
> +       if (unlikely(!card->complete_wq)) {
> +               pr_err("Failed to create mmc completion workqueue");
> +               return -ENOMEM;
> +       }
> +
>         md = mmc_blk_alloc(card);
>         if (IS_ERR(md))
>                 return PTR_ERR(md);
> @@ -2987,6 +2994,7 @@ static void mmc_blk_remove(struct mmc_card *card)
>         pm_runtime_put_noidle(&card->dev);
>         mmc_blk_remove_req(md);
>         dev_set_drvdata(&card->dev, NULL);
> +       destroy_workqueue(card->complete_wq);
>  }
>
>  static int _mmc_blk_suspend(struct mmc_card *card)
> diff --git a/include/linux/mmc/card.h b/include/linux/mmc/card.h
> index de7377815b6b..8ef330027b13 100644
> --- a/include/linux/mmc/card.h
> +++ b/include/linux/mmc/card.h
> @@ -308,6 +308,7 @@ struct mmc_card {
>         unsigned int    nr_parts;
>
>         unsigned int            bouncesz;       /* Bounce buffer size */
> +       struct workqueue_struct *complete_wq;   /* Private workqueue */
>  };
>
>  static inline bool mmc_large_sector(struct mmc_card *card)
> --
> 2.7.4
>