netdev - Re: [PATCH 1/2] Convert target drivers to use sbitmap

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Date:   Mon, 11 Jun 2018 19:18:55 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
        linux1394-devel@...ts.sourceforge.net, linux-usb@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        qla2xxx-upstream@...gic.com,
        Kent Overstreet <kent.overstreet@...il.com>
Cc:     Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 1/2] Convert target drivers to use sbitmap

On 5/15/18 10:11 AM, Jens Axboe wrote:
> On 5/15/18 10:00 AM, Matthew Wilcox wrote:
>> From: Matthew Wilcox <mawilcox@...rosoft.com>
>>
>> The sbitmap and the percpu_ida perform essentially the same task,
>> allocating tags for commands.  Since the sbitmap is more used than
>> the percpu_ida, convert the percpu_ida users to the sbitmap API.
> 
> It should also be the same performance as percpu_ida in light use, and
> performs much better at > 50% utilization of the tag space. I think
> that's better justification than "more used than".
> 
>> diff --git a/drivers/target/iscsi/iscsi_target_util.c b/drivers/target/iscsi/iscsi_target_util.c
>> index 4435bf374d2d..28bcffae609f 100644
>> --- a/drivers/target/iscsi/iscsi_target_util.c
>> +++ b/drivers/target/iscsi/iscsi_target_util.c
>> @@ -17,7 +17,7 @@
>>   ******************************************************************************/
>>  
>>  #include <linux/list.h>
>> -#include <linux/percpu_ida.h>
>> +#include <linux/sched/signal.h>
>>  #include <net/ipv6.h>         /* ipv6_addr_equal() */
>>  #include <scsi/scsi_tcq.h>
>>  #include <scsi/iscsi_proto.h>
>> @@ -147,6 +147,28 @@ void iscsit_free_r2ts_from_list(struct iscsi_cmd *cmd)
>>  	spin_unlock_bh(&cmd->r2t_lock);
>>  }
>>  
>> +int iscsit_wait_for_tag(struct se_session *se_sess, int state, int *cpup)
>> +{
>> +	int tag = -1;
>> +	DEFINE_WAIT(wait);
>> +	struct sbq_wait_state *ws;
>> +
>> +	if (state == TASK_RUNNING)
>> +		return tag;
>> +
>> +	ws = &se_sess->sess_tag_pool.ws[0];
>> +	for (;;) {
>> +		prepare_to_wait_exclusive(&ws->wait, &wait, state);
>> +		if (signal_pending_state(state, current))
>> +			break;
>> +		schedule();
>> +		tag = sbitmap_queue_get(&se_sess->sess_tag_pool, cpup);
>> +	}
>> +
>> +	finish_wait(&ws->wait, &wait);
>> +	return tag;
>> +}
> 
> Seems like that should be:
> 
> 
> 	ws = &se_sess->sess_tag_pool.ws[0];
> 	for (;;) {
> 		prepare_to_wait_exclusive(&ws->wait, &wait, state);
> 		if (signal_pending_state(state, current))
> 			break;
> 		tag = sbitmap_queue_get(&se_sess->sess_tag_pool, cpup);
> 		if (tag != -1)
> 			break;
> 		schedule();
> 	}
> 
> 	finish_wait(&ws->wait, &wait);
> 	return tag;
> 
>>  /*
>>   * May be called from software interrupt (timer) context for allocating
>>   * iSCSI NopINs.
>> @@ -155,9 +177,11 @@ struct iscsi_cmd *iscsit_allocate_cmd(struct iscsi_conn *conn, int state)
>>  {
>>  	struct iscsi_cmd *cmd;
>>  	struct se_session *se_sess = conn->sess->se_sess;
>> -	int size, tag;
>> +	int size, tag, cpu;
>>  
>> -	tag = percpu_ida_alloc(&se_sess->sess_tag_pool, state);
>> +	tag = sbitmap_queue_get(&se_sess->sess_tag_pool, &cpu);
>> +	if (tag < 0)
>> +		tag = iscsit_wait_for_tag(se_sess, state, &cpu);
>>  	if (tag < 0)
>>  		return NULL;
> 
> Might make sense to just roll the whole thing into iscsi_get_tag(), that
> would be cleaner.
> 
> sbitmap should provide a helper for that, but we can do that cleanup
> later. That would encapsulate things like the per-cpu caching hint too,
> for instance.
> 
> Rest looks fine to me.

Are you going to push this further? I really think we should.

-- 
Jens Axboe