Hi Jens, James,
On 2009/03/24 20:42 +0900, Jens Axboe wrote:
> On Wed, Mar 18 2009, James Bottomley wrote:
>> We can eliminate the SCSI command timed out check entirely if the block
>> layer does this for us. The way to do this in block is to check how
>> long the request has been outstanding if a requeue is requested and
>> ending it if we've gone over retries * timeout.
>>
>> This will also eliminate many cases in SCSI where we evade the command
>> timeout for various reasons (like initial success converted to requeue)
>>
>> Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
>>
>> ---
>>
>> Index: linux-2.6/block/blk-core.c
>> ===================================================================
>> --- linux-2.6.orig/block/blk-core.c 2009-02-19 16:42:00.140427205 -0500
>> +++ linux-2.6/block/blk-core.c 2009-03-18 10:36:10.511526815 -0400
>> @@ -937,6 +937,8 @@ EXPORT_SYMBOL(blk_start_queueing);
>> */
>> void blk_requeue_request(struct request_queue *q, struct request *rq)
>> {
>> + unsigned long wait_for = (rq->retries + 1) * rq->timeout;
>> +
>> blk_delete_timer(rq);
>> blk_clear_rq_complete(rq);
>> trace_block_rq_requeue(q, rq);
>> @@ -944,7 +946,13 @@ void blk_requeue_request(struct request_
>> if (blk_rq_tagged(rq))
>> blk_queue_end_tag(q, rq);
>>
>> - elv_requeue_request(q, rq);
>> + if (time_before(rq->start_time + wait_for, jiffies)) {
>> + printk(KERN_ERR "%s: timing out command, waited %lus\n",
>> + rq->rq_disk ? rq->rq_disk->disk_name : "?",
>> + wait_for/HZ);
>> + blk_end_request(rq, -EIO, blk_rq_bytes(rq));
>> + } else
>> + elv_requeue_request(q, rq);
>> }
>> EXPORT_SYMBOL(blk_requeue_request);
>
> Applied, though I think a
>
> time_after(jiffies, time)
>
> is usually much clearer, since we are checking for an expired event (not
> an early one) :-)
I have the following concerns on this patch.
o What if the driver doesn't use the generic timeout handling?
This change means that all block drivers must appropriately set
rq->retries and rq->timeout to use blk_requeue_request(),
although there was no such rule.
So some regressions may be caused in drivers which are not using
rq->retries and rq->timeout for unconditional retry.
(I also need some works in the request-based dm patch for this.)
"!q->rq_timed_out_fn" check in addition to the "time_before()" check
may resolve this issue?
o What if the driver doesn't set rq->retries but has its own logic
to decide when to give up retries?
This change means that drivers which don't use rq->retries
must appropriately modify rq->timeout everytime they call
blk_requeue_request() once timeout fires.
o Deadlock will occur on the blk_end_request() call, because
blk_requeue_request() is called with queue_lock held.
__blk_end_request() needs to be used at least, or other ways are
needed (e.g. throw the request to softirq.)
Thanks,
Kiyoshi Ueda
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