Re: [PATCH 4/6] dm-multipath: remove process_queued_ios()

[Hannes Reinecke <hare@suse.de>]

On 02/03/2014 01:08 PM, Junichi Nomura wrote:
> On 02/03/14 17:18, Hannes Reinecke wrote:
>> We only need to take care to add a small delay when calling
>> __pg_init_all_paths() to move processing off to a workqueue;
>> pg_init_done() is run from an interrupt context and needs to
>> complete as fast as possible.
> ...
>> @@ -1217,9 +1185,11 @@ static void pg_init_done(void *data, int errors)
>>  
>>  	if (!m->pg_init_required)
>>  		m->queue_io = 0;
>> -
>> -	m->pg_init_delay_retry = delay_retry;
>> -	queue_work(kmultipathd, &m->process_queued_ios);
>> +	else {
>> +		m->pg_init_delay_retry = delay_retry;
>> +		__pg_init_all_paths(m, 50/HZ);
>> +		goto out;
>> +	}
>>  
> 
> I forgot to comment on this.
> Adding delay to queue_work() doesn't make it fast.
> So I couldn't see why this "50/HZ" delay has to be added
> and where this value comes.
> 
Well, it wasn't probably the best choice of words.

Thing is, without a delay the workqueue item will be executed
directly (cf __queue_delayed_work()).
But pg_init_done() is run from an interrupt context, and as such any
memory allocations have to be atomic.
So if we were to call queue_delayed_work() without any delay
we will end up calling scsi_dh_activate from an interrupt context,
too, but there we most definitely do _not_ have only atomic allocations.
Hence the delay.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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