Re: [PATCH 1/2] scsi_host: add support for request batching

Paolo Bonzini
2019-06-19T10:32:08Z

On 19/06/19 10:11, Hannes Reinecke wrote:
> On 5/30/19 1:28 PM, Paolo Bonzini wrote:
>> This allows a list of requests to be issued, with the LLD only writing
>> the hardware doorbell when necessary, after the last request was prepared.
>> This is more efficient if we have lists of requests to issue, particularly
>> on virtualized hardware, where writing the doorbell is more expensive than
>> on real hardware.
>>
>> The use case for this is plugged IO, where blk-mq flushes a batch of
>> requests all at once.
>>
>> The API is the same as for blk-mq, just with blk-mq concepts tweaked to
>> fit the SCSI subsystem API: the "last" flag in blk_mq_queue_data becomes
>> a flag in scsi_cmnd, while the queue_num in the commit_rqs callback is
>> extracted from the hctx and passed as a parameter.
>>
>> The only complication is that blk-mq uses different plugging heuristics
>> depending on whether commit_rqs is present or not. So we have two
>> different sets of blk_mq_ops and pick one depending on whether the
>> scsi_host template uses commit_rqs or not.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>> drivers/scsi/scsi_lib.c | 37 ++++++++++++++++++++++++++++++++++---
>> include/scsi/scsi_cmnd.h | 1 +
>> include/scsi/scsi_host.h | 16 ++++++++++++++--
>> 3 files changed, 49 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index 601b9f1de267..eb4e67d02bfe 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -1673,10 +1673,11 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
>> blk_mq_start_request(req);
>> }
>>
>> + cmd->flags &= SCMD_PRESERVED_FLAGS;
>> if (sdev->simple_tags)
>> cmd->flags |= SCMD_TAGGED;
>> - else
>> - cmd->flags &= ~SCMD_TAGGED;
>> + if (bd->last)
>> + cmd->flags |= SCMD_LAST;
>>
>> scsi_init_cmd_errh(cmd);
>> cmd->scsi_done = scsi_mq_done;
>> @@ -1807,10 +1808,37 @@ void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
>> }
>> EXPORT_SYMBOL_GPL(__scsi_init_queue);
>>
>> +static const struct blk_mq_ops scsi_mq_ops_no_commit = {
>> + .get_budget = scsi_mq_get_budget,
>> + .put_budget = scsi_mq_put_budget,
>> + .queue_rq = scsi_queue_rq,
>> + .complete = scsi_softirq_done,
>> + .timeout = scsi_timeout,
>> +#ifdef CONFIG_BLK_DEBUG_FS
>> + .show_rq = scsi_show_rq,
>> +#endif
>> + .init_request = scsi_mq_init_request,
>> + .exit_request = scsi_mq_exit_request,
>> + .initialize_rq_fn = scsi_initialize_rq,
>> + .busy = scsi_mq_lld_busy,
>> + .map_queues = scsi_map_queues,
>> +};
>> +
>> +
>> +static void scsi_commit_rqs(struct blk_mq_hw_ctx *hctx)
>> +{
>> + struct request_queue *q = hctx->queue;
>> + struct scsi_device *sdev = q->queuedata;
>> + struct Scsi_Host *shost = sdev->host;
>> +
>> + shost->hostt->commit_rqs(shost, hctx->queue_num);
>> +}
>> +
>> static const struct blk_mq_ops scsi_mq_ops = {
>> .get_budget = scsi_mq_get_budget,
>> .put_budget = scsi_mq_put_budget,
>> .queue_rq = scsi_queue_rq,
>> + .commit_rqs = scsi_commit_rqs,
>> .complete = scsi_softirq_done,
>> .timeout = scsi_timeout,
>> #ifdef CONFIG_BLK_DEBUG_FS
>> @@ -1845,7 +1873,10 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
>> cmd_size += sizeof(struct scsi_data_buffer) + sgl_size;
>>
>> memset(&shost->tag_set, 0, sizeof(shost->tag_set));
>> - shost->tag_set.ops = &scsi_mq_ops;
>> + if (shost->hostt->commit_rqs)
>> + shost->tag_set.ops = &scsi_mq_ops;
>> + else
>> + shost->tag_set.ops = &scsi_mq_ops_no_commit;
>> shost->tag_set.nr_hw_queues = shost->nr_hw_queues ? : 1;
>> shost->tag_set.queue_depth = shost->can_queue;
>> shost->tag_set.cmd_size = cmd_size;
>> diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
>> index 76ed5e4acd38..91bd749a02f7 100644
>> --- a/include/scsi/scsi_cmnd.h
>> +++ b/include/scsi/scsi_cmnd.h
>> @@ -57,6 +57,7 @@ struct scsi_pointer {
>> #define SCMD_TAGGED (1 << 0)
>> #define SCMD_UNCHECKED_ISA_DMA (1 << 1)
>> #define SCMD_INITIALIZED (1 << 2)
>> +#define SCMD_LAST (1 << 3)
>> /* flags preserved across unprep / reprep */
>> #define SCMD_PRESERVED_FLAGS (SCMD_UNCHECKED_ISA_DMA | SCMD_INITIALIZED)
>>
>> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
>> index 2b539a1b3f62..28f1c9177cd2 100644
>> --- a/include/scsi/scsi_host.h
>> +++ b/include/scsi/scsi_host.h
>> @@ -80,8 +80,10 @@ struct scsi_host_template {
>> * command block to the LLDD. When the driver finished
>> * processing the command the done callback is invoked.
>> *
>> - * If queuecommand returns 0, then the HBA has accepted the
>> - * command. The done() function must be called on the command
>> + * If queuecommand returns 0, then the driver has accepted the
>> + * command. It must also push it to the HBA if the scsi_cmnd
>> + * flag SCMD_LAST is set, or if the driver does not implement
>> + * commit_rqs. The done() function must be called on the command
>> * when the driver has finished with it. (you may call done on the
>> * command before queuecommand returns, but in this case you
>> * *must* return 0 from queuecommand).
>> @@ -109,6 +111,16 @@ struct scsi_host_template {
>> */
>> int (* queuecommand)(struct Scsi_Host *, struct scsi_cmnd *);
>>
>> + /*
>> + * The commit_rqs function is used to trigger a hardware
>> + * doorbell after some requests have been queued with
>> + * queuecommand, when an error is encountered before sending
>> + * the request with SCMD_LAST set.
>> + *
>> + * STATUS: OPTIONAL
>> + */
>> + void (*commit_rqs)(struct Scsi_Host *, u16);
>> +
>> /*
>> * This is an error handling strategy routine. You don't need to
>> * define one of these if you don't want to - there is a default
>>
> I'm a bit unsure if 'bd->last' is always set; it's quite obvious that
> it's present if set, but what about requests with 'bd->last == false' ?
> Is there a guarantee that they will _always_ be followed with a request
> with bd->last == true?
> And if so, is there a guarantee that this request is part of the same batch?

It's complicated. A request with bd->last == false _will_ always be
followed by a request with bd->last == true in the same batch. However,
due to e.g. errors it may be possible that the last request is not sent.
In that case, the block layer sends commit_rqs, as documented in the
comment above, to flush the requests that have been sent already.

So, a driver that obeys bd->last (or SCMD_LAST) but does not implement
commit_rqs is bound to have bugs, which is why this patch was not split
further.

Makes sense?

Paolo

> Aside from it: I think it's a good idea to match the '->last' setting
> onto the SCMD_LAST flag; I would even go so far and make this an
> independent patch.

> Once to above points are cleared, that is.
>
> But if that one is in, why do we need to have the separate 'commit_rqs'
> callback?
> Can't we let the driver decide to issue a doorbell kick (or whatever the
> driver decides to do there)?
> If we ensure that the SCMD_LAST flag is always set for the end of a
> batch (even if this batch consists only of one request), the driver
> simply can evaluate the flag and do its actions.
> Why do we need a new callback here?
>
> Cheers,
>
> Hannes
>