[0/5] blk-mq/scsi-mq: support global tags & introduce force_blk_mq

> -----Original Message-----
> From: Ming Lei [mailto:ming.lei@redhat.com]
> Sent: Tuesday, February 6, 2018 6:02 PM
> To: Kashyap Desai
> Cc: Hannes Reinecke; Jens Axboe; linux-block@vger.kernel.org; Christoph
> Hellwig; Mike Snitzer; linux-scsi@vger.kernel.org; Arun Easi; Omar
Sandoval;
> Martin K . Petersen; James Bottomley; Christoph Hellwig; Don Brace;
Peter
> Rivera; Paolo Bonzini; Laurence Oberman
> Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global tags & introduce
> force_blk_mq
>
> Hi Kashyap,
>
> On Tue, Feb 06, 2018 at 04:59:51PM +0530, Kashyap Desai wrote:
> > > -----Original Message-----
> > > From: Ming Lei [mailto:ming.lei@redhat.com]
> > > Sent: Tuesday, February 6, 2018 1:35 PM
> > > To: Kashyap Desai
> > > Cc: Hannes Reinecke; Jens Axboe; linux-block@vger.kernel.org;
> > > Christoph Hellwig; Mike Snitzer; linux-scsi@vger.kernel.org; Arun
> > > Easi; Omar
> > Sandoval;
> > > Martin K . Petersen; James Bottomley; Christoph Hellwig; Don Brace;
> > Peter
> > > Rivera; Paolo Bonzini; Laurence Oberman
> > > Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global tags &
> > > introduce force_blk_mq
> > >
> > > Hi Kashyap,
> > >
> > > On Tue, Feb 06, 2018 at 11:33:50AM +0530, Kashyap Desai wrote:
> > > > > > We still have more than one reply queue ending up completion
> > > > > > one
> > CPU.
> > > > >
> > > > > pci_alloc_irq_vectors(PCI_IRQ_AFFINITY) has to be used, that
> > > > > means smp_affinity_enable has to be set as 1, but seems it is
> > > > > the default
> > > > setting.
> > > > >
> > > > > Please see kernel/irq/affinity.c, especially
> > > > > irq_calc_affinity_vectors()
> > > > which
> > > > > figures out an optimal number of vectors, and the computation is
> > > > > based
> > > > on
> > > > > cpumask_weight(cpu_possible_mask) now. If all offline CPUs are
> > > > > mapped to some of reply queues, these queues won't be active(no
> > > > > request submitted
> > > > to
> > > > > these queues). The mechanism of PCI_IRQ_AFFINITY basically makes
> > > > > sure
> > > > that
> > > > > more than one irq vector won't be handled by one same CPU, and
> > > > > the irq vector spread is done in irq_create_affinity_masks().
> > > > >
> > > > > > Try to reduce MSI-x vector of megaraid_sas or mpt3sas driver
> > > > > > via module parameter to simulate the issue. We need more
> > > > > > number of Online CPU than reply-queue.
> > > > >
> > > > > IMO, you don't need to simulate the issue,
> > > > > pci_alloc_irq_vectors(
> > > > > PCI_IRQ_AFFINITY) will handle that for you. You can dump the
> > > > > returned
> > > > irq
> > > > > vector number, num_possible_cpus()/num_online_cpus() and each
> > > > > irq vector's affinity assignment.
> > > > >
> > > > > > We may see completion redirected to original CPU because of
> > > > > > "QUEUE_FLAG_SAME_FORCE", but ISR of low level driver can keep
> > > > > > one CPU busy in local ISR routine.
> > > > >
> > > > > Could you dump each irq vector's affinity assignment of your
> > > > > megaraid in
> > > > your
> > > > > test?
> > > >
> > > > To quickly reproduce, I restricted to single MSI-x vector on
> > > > megaraid_sas driver.  System has total 16 online CPUs.
> > >
> > > I suggest you don't do the restriction of single MSI-x vector, and
> > > just
> > use the
> > > device supported number of msi-x vector.
> >
> > Hi Ming,  CPU lock up is seen even though it is not single msi-x
vector.
> > Actual scenario need some specific topology and server for overnight
test.
> > Issue can be seen on servers which has more than 16 logical CPUs and
> > Thunderbolt series MR controller which supports at max 16 MSIx
vectors.
> > >
> > > >
> > > > Output of affinity hints.
> > > > kernel version:
> > > > Linux rhel7.3 4.15.0-rc1+ #2 SMP Mon Feb 5 12:13:34 EST 2018
> > > > x86_64
> > > > x86_64
> > > > x86_64 GNU/Linux
> > > > PCI name is 83:00.0, dump its irq affinity:
> > > > irq 105, cpu list 0-3,8-11
> > >
> > > In this case, which CPU is selected for handling the interrupt is
> > decided by
> > > interrupt controller, and it is easy to cause CPU overload if
> > > interrupt
> > controller
> > > always selects one same CPU to handle the irq.
> > >
> > > >
> > > > Affinity mask is created properly, but only CPU-0 is overloaded
> > > > with interrupt processing.
> > > >
> > > > # numactl --hardware
> > > > available: 2 nodes (0-1)
> > > > node 0 cpus: 0 1 2 3 8 9 10 11
> > > > node 0 size: 47861 MB
> > > > node 0 free: 46516 MB
> > > > node 1 cpus: 4 5 6 7 12 13 14 15
> > > > node 1 size: 64491 MB
> > > > node 1 free: 62805 MB
> > > > node distances:
> > > > node   0   1
> > > >   0:  10  21
> > > >   1:  21  10
> > > >
> > > > Output of  system activities (sar).  (gnice is 100% and it is
> > > > consumed in megaraid_sas ISR routine.)
> > > >
> > > >
> > > > 12:44:40 PM     CPU      %usr     %nice      %sys   %iowait
%steal
> > > > %irq     %soft    %guest    %gnice     %idle
> > > > 12:44:41 PM     all         6.03      0.00        29.98      0.00
> > > > 0.00         0.00        0.00        0.00        0.00
63.99
> > > > 12:44:41 PM       0         0.00      0.00         0.00
0.00
> > > > 0.00         0.00        0.00        0.00       100.00         0
> > > >
> > > >
> > > > In my test, I used rq_affinity is set to 2.
> > > > (QUEUE_FLAG_SAME_FORCE). I also used " host_tagset" V2 patch set
for
> megaraid_sas.
> > > >
> > > > Using RFC requested in -
> > > > "https://marc.info/?l=linux-scsi&m=151601833418346&w=2 " lockup is
> > > > avoided (you can noticed that gnice is shifted to softirq. Even
> > > > though it is 100% consumed, There is always exit for existing
> > > > completion loop due to irqpoll_weight  @irq_poll_init().
> > > >
> > > > Average:        CPU      %usr     %nice      %sys   %iowait
%steal
> > > > %irq     %soft    %guest    %gnice     %idle
> > > > Average:        all          4.25      0.00        21.61      0.00
> > > > 0.00      0.00         6.61           0.00      0.00     67.54
> > > > Average:          0           0.00      0.00         0.00
0.00
> > > > 0.00      0.00       100.00        0.00      0.00      0.00
> > > >
> > > >
> > > > Hope this clarifies. We need different fix to avoid lockups. Can
> > > > we consider using irq poll interface if #CPU is more than Reply
> > queue/MSI-x.
> > > > ?
> > >
> > > Please use the device's supported msi-x vectors number, and see if
> > > there
> > is this
> > > issue. If there is, you can use irq poll too, which isn't
> > > contradictory
> > with the
> > > blk-mq approach taken by this patchset.
> >
> > Device supported scenario need more time to reproduce, but it is more
> > quick method is to just use single MSI-x vector and try to create
> > worst case IO completion loop.
> > Using irq poll, my test run without any CPU lockup. I tried your
> > latest V2 series as well and that is also behaving the same.
>
> Again, you can use irq poll, which isn't contradictory with blk-mq.

Just wanted to explained that issue of CPU lock up is different.  Thanks
for clarification.
>
> >
> > BTW - I am seeing drastically performance drop using V2 series of
> > patch on megaraid_sas. Those who is testing HPSA, can also verify if
> > that is a generic behavior.
>
> OK, I will see if I can find a megaraid_sas to see the performance drop
issue. If I
> can't, I will try to run performance test on HPSA.

Patch is appended.

>
> Could you share us your patch for enabling global_tags/MQ on
megaraid_sas
> so that I can reproduce your test?
>
> > See below perf top data. "bt_iter" is consuming 4 times more CPU.
>
> Could you share us what the IOPS/CPU utilization effect is after
applying the
> patch V2? And your test script?
Regarding CPU utilization, I need to test one more time. Currently system
is in used.

I run below fio test on total 24 SSDs expander attached.

numactl -N 1 fio jbod.fio --rw=randread --iodepth=64 --bs=4k
--ioengine=libaio --rw=randread

Performance dropped from 1.6 M IOPs to 770K IOPs.

>
> In theory, it shouldn't, because the HBA only supports HBA wide tags,
that
> means the allocation has to share a HBA wide sbitmap no matter if global
tags
> is used or not.
>
> Anyway, I will take a look at the performance test and data.
>
>
> Thanks,
> Ming


Megaraid_sas version of shared tag set.


+	cmd->request_desc->SCSIIO.MSIxIndex =
blk_mq_unique_tag_to_hwq(scp->request->tag);

 	praid_context = &io_request->RaidContext;

@@ -2985,9 +2989,13 @@ static void megasas_build_ld_nonrw_fusion(struct
megasas_instance *instance,
 	}

 	cmd->request_desc->SCSIIO.DevHandle = io_request->DevHandle;
+
+#if 0
 	cmd->request_desc->SCSIIO.MSIxIndex =
 		instance->msix_vectors ?
 		(raw_smp_processor_id() % instance->msix_vectors) : 0;
+#endif
+	cmd->request_desc->SCSIIO.MSIxIndex =
blk_mq_unique_tag_to_hwq(scmd->request->tag);


 	if (!fp_possible) {


[0/5] blk-mq/scsi-mq: support global tags & introduce force_blk_mq

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