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

Ming Lei
2018-02-06T15:46:32Z

Hi Kashyap,

On Tue, Feb 06, 2018 at 07:57:35PM +0530, Kashyap Desai wrote:
> > -----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.

Thanks for providing the patch.

>
>
> diff --git a/drivers/scsi/megaraid/megaraid_sas_base.c
> b/drivers/scsi/megaraid/megaraid_sas_base.c
> index 0f1d88f..75ea86b 100644
> --- a/drivers/scsi/megaraid/megaraid_sas_base.c
> +++ b/drivers/scsi/megaraid/megaraid_sas_base.c
> @@ -50,6 +50,7 @@
> #include <linux/mutex.h>
> #include <linux/poll.h>
> #include <linux/vmalloc.h>
> +#include <linux/blk-mq-pci.h>
>
> #include <scsi/scsi.h>
> #include <scsi/scsi_cmnd.h>
> @@ -220,6 +221,15 @@ static int megasas_get_ld_vf_affiliation(struct
> megasas_instance *instance,
> static inline void
> megasas_init_ctrl_params(struct megasas_instance *instance);
>
> +
> +static int megaraid_sas_map_queues(struct Scsi_Host *shost)
> +{
> + struct megasas_instance *instance;
> + instance = (struct megasas_instance *)shost->hostdata;
> +
> + return blk_mq_pci_map_queues(&shost->tag_set, instance->pdev);
> +}
> +
> /**
> * megasas_set_dma_settings - Populate DMA address, length and flags for
> DCMDs
> * @instance: Adapter soft state
> @@ -3177,6 +3187,8 @@ struct device_attribute *megaraid_host_attrs[] = {
> .use_clustering = ENABLE_CLUSTERING,
> .change_queue_depth = scsi_change_queue_depth,
> .no_write_same = 1,
> + .map_queues = megaraid_sas_map_queues,
> + .host_tagset = 1,
> };
>
> /**
> @@ -5965,6 +5977,9 @@ static int megasas_io_attach(struct megasas_instance
> *instance)
> host->max_lun = MEGASAS_MAX_LUN;
> host->max_cmd_len = 16;
>
> + /* map reply queue to blk_mq hw queue */
> + host->nr_hw_queues = instance->msix_vectors;
> +
> /*
> * Notify the mid-layer about the new controller
> */
> diff --git a/drivers/scsi/megaraid/megaraid_sas_fusion.c
> b/drivers/scsi/megaraid/megaraid_sas_fusion.c
> index 073ced0..034d976 100644
> --- a/drivers/scsi/megaraid/megaraid_sas_fusion.c
> +++ b/drivers/scsi/megaraid/megaraid_sas_fusion.c
> @@ -2655,11 +2655,15 @@ static void megasas_stream_detect(struct
> megasas_instance *instance,
> fp_possible = (io_info.fpOkForIo > 0) ? true :
> false;
> }
>
> +#if 0
> /* Use raw_smp_processor_id() for now until cmd->request->cpu is
> CPU
> id by default, not CPU group id, otherwise all MSI-X queues
> won't
> be utilized */
> 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(scp->request->tag);

Looks the above line is wrong, and you just use reply queue 0 in this way
to complete rq.

Follows the correct usage:

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

>
> 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);

Same with above, could you fix the patch and run your performance test
again?

Thanks
Ming