Re: [PATCH v3 20/77] ncr5380: Introduce unbound workqueue

Hannes Reinecke
2015-12-22T14:48:24Z

On 12/22/2015 01:44 PM, Finn Thain wrote:
>
> On Tue, 22 Dec 2015, Hannes Reinecke wrote:
>
>> On 12/22/2015 02:17 AM, Finn Thain wrote:
>>> Allocate a work queue that will permit busy waiting and sleeping. This
>>> means NCR5380_init() can potentially fail, so add this error path.
>>>
>>> Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
>>>
>>> ---
>>>
>>> In subsequent patches, the work function adopts this work queue so it
>>> can sleep while polling, which allows the removal of some flawed and
>>> complicated code in NCR5380_select() in NCR5380.c.
>>>
>>> Changed since v1:
>>> - Dropped WQ_CPU_INTENSIVE flag because Documentation/workqueue.txt says it
>>> "is meaningless for unbound wq".
>>>
>>> ---
>>> drivers/scsi/NCR5380.c | 15 +++++++++++----
>>> drivers/scsi/NCR5380.h | 1 +
>>> drivers/scsi/arm/cumana_1.c | 8 ++++++--
>>> drivers/scsi/arm/oak.c | 8 ++++++--
>>> drivers/scsi/atari_NCR5380.c | 8 +++++++-
>>> drivers/scsi/atari_scsi.c | 5 ++++-
>>> drivers/scsi/dmx3191d.c | 17 +++++++++++------
>>> drivers/scsi/dtc.c | 11 +++++++++--
>>> drivers/scsi/g_NCR5380.c | 31 +++++++++++++++----------------
>>> drivers/scsi/mac_scsi.c | 5 ++++-
>>> drivers/scsi/pas16.c | 10 ++++++++--
>>> drivers/scsi/sun3_scsi.c | 5 ++++-
>>> drivers/scsi/t128.c | 13 ++++++++++---
>>> 13 files changed, 96 insertions(+), 41 deletions(-)
>>>
>>> Index: linux/drivers/scsi/NCR5380.c
>>> ===================================================================
>>> --- linux.orig/drivers/scsi/NCR5380.c 2015-12-22 12:15:52.000000000 +1100
>>> +++ linux/drivers/scsi/NCR5380.c 2015-12-22 12:15:56.000000000 +1100
>>> @@ -514,7 +514,7 @@ static int should_disconnect(unsigned ch
>>> static void NCR5380_set_timer(struct NCR5380_hostdata *hostdata, unsigned
>>> long timeout)
>>> {
>>> hostdata->time_expires = jiffies + timeout;
>>> - schedule_delayed_work(&hostdata->coroutine, timeout);
>>> + queue_delayed_work(hostdata->work_q, &hostdata->coroutine, timeout);
>>> }
>>>
>>>
>>> @@ -791,7 +791,12 @@ static int NCR5380_init(struct Scsi_Host
>>> hostdata->disconnected_queue = NULL;
>>>
>>> INIT_DELAYED_WORK(&hostdata->coroutine, NCR5380_main);
>>> -
>>> + hostdata->work_q = alloc_workqueue("ncr5380_%d",
>>> + WQ_UNBOUND | WQ_MEM_RECLAIM,
>>> + 1, instance->host_no);
>>> + if (!hostdata->work_q)
>>> + return -ENOMEM;
>>> +
>>> /* The CHECK code seems to break the 53C400. Will check it later maybe */
>>> if (flags & FLAG_NCR53C400)
>>> hostdata->flags = FLAG_HAS_LAST_BYTE_SENT | flags;
>>
>> Wouldn't it be better to use a normal (ie bound) workqueue here?
>
> The polling algorithm I've used requires that the workqueue item is free
> to busy-wait and sleep. Perhaps a kthread_worker would be better?
>
>> SCSI-2 is pretty much single-threaded, so shifting things onto arbitrary
>> CPUs don't sound very appealing.
>
> Most of these drivers only run on UP systems. For the x86 drivers, I
> suspect that the cache miss penalty would be insignificant compared to
> some of the other overheads. The 5380 chip requires that the CPU is
> involved in SCSI bus signalling and merely accessing a chip register
> takes over a microsecond.
>
I know.
But using a bound workqueue would mean you could use
'create_workqueue()' instead of open-coding it :-)

But in the end it's up to you. If the thing works I'm not that concerned.

Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare@suse.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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