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Date: Mon, 30 Jan 2017 19:22:03 +0530
From: Kashyap Desai <kashyap.desai@...adcom.com>
To: Jens Axboe <axboe@...nel.dk>, Omar Sandoval <osandov@...ndov.com>
Cc: linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
paolo.valente@...aro.org
Subject: RE: Device or HBA level QD throttling creates randomness in sequetial workload
Hi Jens/Omar,
I used git.kernel.dk/linux-block branch - blk-mq-sched (commit
0efe27068ecf37ece2728a99b863763286049ab5) and confirm that issue reported in
this thread is resolved.
Now I am seeing MQ and SQ mode both are resulting in sequential IO pattern
while IO is getting re-queued in block layer.
To make similar performance without blk-mq-sched feature, is it good to
pause IO for few usec in LLD?
I mean, I want to avoid driver asking SML/Block layer to re-queue the IO (if
it is Sequential on Rotational media.)
Explaining w.r.t megaraid_sas driver. This driver expose can_queue, but it
internally consume commands for raid 1, fast path.
In worst case, can_queue/2 will consume all firmware resources and driver
will re-queue further IOs to SML as below -
if (atomic_inc_return(&instance->fw_outstanding) >
instance->host->can_queue) {
atomic_dec(&instance->fw_outstanding);
return SCSI_MLQUEUE_HOST_BUSY;
}
I want to avoid above SCSI_MLQUEUE_HOST_BUSY.
Need your suggestion for below changes -
diff --git a/drivers/scsi/megaraid/megaraid_sas_fusion.c
b/drivers/scsi/megaraid/megaraid_sas_fusion.c
index 9a9c84f..a683eb0 100644
--- a/drivers/scsi/megaraid/megaraid_sas_fusion.c
+++ b/drivers/scsi/megaraid/megaraid_sas_fusion.c
@@ -54,6 +54,7 @@
#include <scsi/scsi_host.h>
#include <scsi/scsi_dbg.h>
#include <linux/dmi.h>
+#include <linux/cpumask.h>
#include "megaraid_sas_fusion.h"
#include "megaraid_sas.h"
@@ -2572,7 +2573,15 @@ void megasas_prepare_secondRaid1_IO(struct
megasas_instance *instance,
struct megasas_cmd_fusion *cmd, *r1_cmd = NULL;
union MEGASAS_REQUEST_DESCRIPTOR_UNION *req_desc;
u32 index;
- struct fusion_context *fusion;
+ bool is_nonrot;
+ u32 safe_can_queue;
+ u32 num_cpus;
+ struct fusion_context *fusion;
+
+ fusion = instance->ctrl_context;
+
+ num_cpus = num_online_cpus();
+ safe_can_queue = instance->cur_can_queue - num_cpus;
fusion = instance->ctrl_context;
@@ -2584,11 +2593,15 @@ void megasas_prepare_secondRaid1_IO(struct
megasas_instance *instance,
return SCSI_MLQUEUE_DEVICE_BUSY;
}
- if (atomic_inc_return(&instance->fw_outstanding) >
- instance->host->can_queue) {
- atomic_dec(&instance->fw_outstanding);
- return SCSI_MLQUEUE_HOST_BUSY;
- }
+ if (atomic_inc_return(&instance->fw_outstanding) > safe_can_queue) {
+ is_nonrot = blk_queue_nonrot(scmd->device->request_queue);
+ /* For rotational device wait for sometime to get fusion command
from pool.
+ * This is just to reduce proactive re-queue at mid layer which is
not
+ * sending sorted IO in SCSI.MQ mode.
+ */
+ if (!is_nonrot)
+ udelay(100);
+ }
cmd = megasas_get_cmd_fusion(instance, scmd->request->tag);
` Kashyap
> -----Original Message-----
> From: Kashyap Desai [mailto:kashyap.desai@...adcom.com]
> Sent: Tuesday, November 01, 2016 11:11 AM
> To: 'Jens Axboe'; 'Omar Sandoval'
> Cc: 'linux-scsi@...r.kernel.org'; 'linux-kernel@...r.kernel.org'; 'linux-
> block@...r.kernel.org'; 'Christoph Hellwig'; 'paolo.valente@...aro.org'
> Subject: RE: Device or HBA level QD throttling creates randomness in
> sequetial workload
>
> Jens- Replied inline.
>
>
> Omar - I tested your WIP repo and figure out System hangs only if I pass
> "
> scsi_mod.use_blk_mq=Y". Without this, your WIP branch works fine, but I
> am looking for scsi_mod.use_blk_mq=Y.
>
> Also below is snippet of blktrace. In case of higher per device QD, I see
> Requeue request in blktrace.
>
> 65,128 10 6268 2.432404509 18594 P N [fio]
> 65,128 10 6269 2.432405013 18594 U N [fio] 1
> 65,128 10 6270 2.432405143 18594 I WS 148800 + 8 [fio]
> 65,128 10 6271 2.432405740 18594 R WS 148800 + 8 [0]
> 65,128 10 6272 2.432409794 18594 Q WS 148808 + 8 [fio]
> 65,128 10 6273 2.432410234 18594 G WS 148808 + 8 [fio]
> 65,128 10 6274 2.432410424 18594 S WS 148808 + 8 [fio]
> 65,128 23 3626 2.432432595 16232 D WS 148800 + 8
> [kworker/23:1H]
> 65,128 22 3279 2.432973482 0 C WS 147432 + 8 [0]
> 65,128 7 6126 2.433032637 18594 P N [fio]
> 65,128 7 6127 2.433033204 18594 U N [fio] 1
> 65,128 7 6128 2.433033346 18594 I WS 148808 + 8 [fio]
> 65,128 7 6129 2.433033871 18594 D WS 148808 + 8 [fio]
> 65,128 7 6130 2.433034559 18594 R WS 148808 + 8 [0]
> 65,128 7 6131 2.433039796 18594 Q WS 148816 + 8 [fio]
> 65,128 7 6132 2.433040206 18594 G WS 148816 + 8 [fio]
> 65,128 7 6133 2.433040351 18594 S WS 148816 + 8 [fio]
> 65,128 9 6392 2.433133729 0 C WS 147240 + 8 [0]
> 65,128 9 6393 2.433138166 905 D WS 148808 + 8 [kworker/9:1H]
> 65,128 7 6134 2.433167450 18594 P N [fio]
> 65,128 7 6135 2.433167911 18594 U N [fio] 1
> 65,128 7 6136 2.433168074 18594 I WS 148816 + 8 [fio]
> 65,128 7 6137 2.433168492 18594 D WS 148816 + 8 [fio]
> 65,128 7 6138 2.433174016 18594 Q WS 148824 + 8 [fio]
> 65,128 7 6139 2.433174282 18594 G WS 148824 + 8 [fio]
> 65,128 7 6140 2.433174613 18594 S WS 148824 + 8 [fio]
> CPU0 (sdy):
> Reads Queued: 0, 0KiB Writes Queued: 79,
> 316KiB
> Read Dispatches: 0, 0KiB Write Dispatches: 67,
> 18,446,744,073PiB
> Reads Requeued: 0 Writes Requeued: 86
> Reads Completed: 0, 0KiB Writes Completed: 98,
> 392KiB
> Read Merges: 0, 0KiB Write Merges: 0,
> 0KiB
> Read depth: 0 Write depth: 5
> IO unplugs: 79 Timer unplugs: 0
>
>
>
> ` Kashyap
>
> > -----Original Message-----
> > From: Jens Axboe [mailto:axboe@...nel.dk]
> > Sent: Monday, October 31, 2016 10:54 PM
> > To: Kashyap Desai; Omar Sandoval
> > Cc: linux-scsi@...r.kernel.org; linux-kernel@...r.kernel.org; linux-
> > block@...r.kernel.org; Christoph Hellwig; paolo.valente@...aro.org
> > Subject: Re: Device or HBA level QD throttling creates randomness in
> > sequetial workload
> >
> > Hi,
> >
> > One guess would be that this isn't around a requeue condition, but
> > rather the fact that we don't really guarantee any sort of hard FIFO
> > behavior between the software queues. Can you try this test patch to
> > see if it changes the behavior for you? Warning: untested...
>
> Jens - I tested the patch, but I still see random IO pattern for expected
> Sequential Run. I am intentionally running case of Re-queue and seeing
> issue at the time of Re-queue.
> If there is no Requeue, I see no issue at LLD.
>
>
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c index
> > f3d27a6dee09..5404ca9c71b2
> > 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -772,6 +772,14 @@ static inline unsigned int
> > queued_to_index(unsigned int
> > queued)
> > return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
> > }
> >
> > +static int rq_pos_cmp(void *priv, struct list_head *a, struct
> > +list_head
> > +*b) {
> > + struct request *rqa = container_of(a, struct request, queuelist);
> > + struct request *rqb = container_of(b, struct request, queuelist);
> > +
> > + return blk_rq_pos(rqa) < blk_rq_pos(rqb); }
> > +
> > /*
> > * Run this hardware queue, pulling any software queues mapped to it
> > in.
> > * Note that this function currently has various problems around
> > ordering @@ -
> > 812,6 +820,14 @@ static void __blk_mq_run_hw_queue(struct
> > blk_mq_hw_ctx
> > *hctx)
> > }
> >
> > /*
> > + * If the device is rotational, sort the list sanely to avoid
> > + * unecessary seeks. The software queues are roughly FIFO, but
> > + * only roughly, there are no hard guarantees.
> > + */
> > + if (!blk_queue_nonrot(q))
> > + list_sort(NULL, &rq_list, rq_pos_cmp);
> > +
> > + /*
> > * Start off with dptr being NULL, so we start the first request
> > * immediately, even if we have more pending.
> > */
> >
> > --
> > Jens Axboe
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