Observing Softlockup's while running heavy IOs

Sreekanth Reddy sreekanth.reddy at broadcom.com
Wed Sep 7 22:42:40 PDT 2016


On Wed, Sep 7, 2016 at 6:54 PM, Neil Horman <nhorman at tuxdriver.com> wrote:
> On Wed, Sep 07, 2016 at 11:30:04AM +0530, Sreekanth Reddy wrote:
>> On Tue, Sep 6, 2016 at 8:36 PM, Neil Horman <nhorman at tuxdriver.com> wrote:
>> > On Tue, Sep 06, 2016 at 04:52:37PM +0530, Sreekanth Reddy wrote:
>> >> On Fri, Sep 2, 2016 at 4:34 AM, Bart Van Assche
>> >> <bart.vanassche at sandisk.com> wrote:
>> >> > On 09/01/2016 03:31 AM, Sreekanth Reddy wrote:
>> >> >>
>> >> >> I reduced the ISR workload by one third in-order to reduce the time
>> >> >> that is spent per CPU in interrupt context, even then I am observing
>> >> >> softlockups.
>> >> >>
>> >> >> As I mentioned before only same single CPU in the set of CPUs(enabled
>> >> >> in affinity_hint) is busy with handling the interrupts from
>> >> >> corresponding IRQx. I have done below experiment in driver to limit
>> >> >> these softlockups/hardlockups. But I am not sure whether it is
>> >> >> reasonable to do this in driver,
>> >> >>
>> >> >> Experiment:
>> >> >> If the CPUx is continuously busy with handling the remote CPUs
>> >> >> (enabled in the corresponding IRQ's affinity_hint) IO works by 1/4th
>> >> >> of the HBA queue depth in the same ISR context then enable a flag
>> >> >> called 'change_smp_affinity' for this IRQ. Also created a thread with
>> >> >> will poll for this flag for every IRQ's (enabled by driver) for every
>> >> >> second. If this thread see that this flag is enabled for any IRQ then
>> >> >> it will write next CPU number from the CPUs enabled in the IRQ's
>> >> >> affinity_hint to the IRQ's smp_affinity procfs attribute using
>> >> >> 'call_usermodehelper()' API.
>> >> >>
>> >> >> This to make sure that interrupts are not processed by same single CPU
>> >> >> all the time and to make the other CPUs to handle the interrupts if
>> >> >> the current CPU is continuously busy with handling the other CPUs IO
>> >> >> interrupts.
>> >> >>
>> >> >> For example consider a system which has 8 logical CPUs and one MSIx
>> >> >> vector enabled (called IRQ 120) in driver, HBA queue depth as 8K.
>> >> >> then IRQ's procfs attributes will be
>> >> >> IRQ# 120, affinity_hint=0xff, smp_affinity=0x00
>> >> >>
>> >> >> After starting heavy IOs, we will observe that only CPU0 will be busy
>> >> >> with handling the interrupts. This experiment driver will change the
>> >> >> smp_affinity to next CPU number i.e. 0x01 (using cmd 'echo 0x01 >
>> >> >> /proc/irq/120/smp_affinity', driver issue's this cmd using
>> >> >> call_usermodehelper() API) if it observes that CPU0 is continuously
>> >> >> processing more than 2K of IOs replies of other CPUs i.e from CPU1 to
>> >> >> CPU7.
>> >> >>
>> >> >> Whether doing this kind of stuff in driver is ok?
>> >> >
>> >> >
>> >> > Hello Sreekanth,
>> >> >
>> >> > To me this sounds like something that should be implemented in the I/O
>> >> > chipset on the motherboard. If you have a look at the Intel Software
>> >> > Developer Manuals then you will see that logical destination mode supports
>> >> > round-robin interrupt delivery. However, the Linux kernel selects physical
>> >> > destination mode on systems with more than eight logical CPUs (see also
>> >> > arch/x86/kernel/apic/apic_flat_64.c).
>> >> >
>> >> > I'm not sure the maintainers of the interrupt subsystem would welcome code
>> >> > that emulates round-robin interrupt delivery. So your best option is
>> >> > probably to minimize the amount of work that is done in interrupt context
>> >> > and to move as much work as possible out of interrupt context in such a way
>> >> > that it can be spread over multiple CPU cores, e.g. by using
>> >> > queue_work_on().
>> >> >
>> >> > Bart.
>> >>
>> >> Bart,
>> >>
>> >> Thanks a lot for providing lot of inputs and valuable information on this issue.
>> >>
>> >> Today I got one more observation. i.e. I am not observing any lockups
>> >> if I use 1.0.4-6 versioned irqbalance.
>> >> Since this versioned irqbalance is able to shift the load to other CPU
>> >> when one CPU is heavily loaded.
>> >>
>> >
>> > This isn't happening because irqbalance is no longer able to shift load between
>> > cpus, its happening because of commit 996ee2cf7a4d10454de68ac4978adb5cf22850f8.
>> > irqs with higher interrupt volumes sould be balanced to a specific cpu core,
>> > rather than to a cache domain to maximize cpu-local cache hit rates.  Prior to
>> > that change we balanced to a cache domain and your workload didn't have to
>> > serialize multiple interrupts to a single core.  My suggestion to you is to use
>> > the --policyscript option to make your storage irqs get balanced to the cache
>> > level, rather than the core level.  That should return the behavior to what you
>> > want.
>> >
>> > Neil
>>
>> Hi Neil,
>>
>> Thanks for reply.
>>
>> Today I tried with setting balance_level to 'cache' for mpt3sas driver
>> IRQ's using below policy script and used 1.0.9 versioned irqbalance,
>> ----------------------------------------------------------------------------------------------
>> #!/bin/bash
>> # Header
>> # Linux Shell Scripting for Irq Balance Policy select for mpt3sas driver
>> #
>>
>> # Command Line Args
>>  #IRQ_PATH    -> PATH
>>  #IRQ_NUMBER     -> IRQ Number
>> declare -r IRQ_PATH=$1
>> declare -r IRQ_NUMBER=$2
>>
>> if [ -d /proc/irq/$IRQ_NUMBER ]; then
>>         mpt3sas_irq=(`ls /proc/irq/$IRQ_NUMBER/ | grep mpt3sas | wc -l`)
>>         if [ $mpt3sas_irq == 1 ]; then
>>             echo "hintpolicy=subset"
>>             echo "balance_level=cache"
>>     fi
>> fi
>> -----------------------------------------------------------------------------------------------
>>
>> But still I don't see any load shift happening between the CPUs and
>> still observing hardlockups.
>>
>> Here I have attached the irqbalance logs.
>>
>> Thanks,
>> Sreekanth
>
> Hey there-
>         So, looking at your logs, your script is working correctly:
> Package 0:  numa_node is 0 cpu mask is 0003f03f (load 0)
>         Cache domain 0:  numa_node is 0 cpu mask is 00001001  (load 0)
>                 CPU number 0  numa_node is 0 (load 0)
>                   Interrupt 150 node_num is 0 (storage/1)
>                   Interrupt 174 node_num is 0 (storage/1)
>                   Interrupt 198 node_num is 0 (storage/1)
>                   Interrupt 126 node_num is 0 (storage/1)
>                   Interrupt 102 node_num is 0 (ethernet/1)
>                   Interrupt 77 node_num is 0 (ethernet/1)
>                 CPU number 12  numa_node is 0 (load 0)
>                   Interrupt 138 node_num is 0 (storage/1)
>                   Interrupt 162 node_num is 0 (storage/1)
>                   Interrupt 186 node_num is 0 (storage/1)
>                   Interrupt 114 node_num is 0 (storage/1)
>                   Interrupt 90 node_num is 0 (ethernet/1)
>                   Interrupt 65 node_num is 0 (ethernet/1)
>           Interrupt 51 node_num is -1 (storage/1)
>           Interrupt 31 node_num is 0 (legacy/1)
> ...
> Package 1:  numa_node is 0 cpu mask is 00fc0fc0 (load 0)
>         Cache domain 6:  numa_node is 0 cpu mask is 00040040  (load 0)
>                 CPU number 6  numa_node is 0 (load 0)
>                   Interrupt 149 node_num is 0 (storage/1)
>                   Interrupt 173 node_num is 0 (storage/1)
>                   Interrupt 197 node_num is 0 (storage/1)
>                   Interrupt 125 node_num is 0 (storage/1)
>                   Interrupt 101 node_num is 0 (ethernet/1)
>                   Interrupt 76 node_num is 0 (ethernet/1)
>                 CPU number 18  numa_node is 0 (load 0)
>                   Interrupt 137 node_num is 0 (storage/1)
>                   Interrupt 161 node_num is 0 (storage/1)
>                   Interrupt 185 node_num is 0 (storage/1)
>                   Interrupt 113 node_num is 0 (storage/1)
>                   Interrupt 89 node_num is 0 (ethernet/1)
>                   Interrupt 64 node_num is 0 (ethernet/1)
>           Interrupt 50 node_num is -1 (storage/1)
>
>
> irqbalance correctly decided to balance irqs 50 and 51 to the cache level, which
> is good. The only other thing I would check though is the affinity_hint those
> irqs are exporting.  With an affinity hint set to subset, if the exported hint
> only intersects the cache domain cpu set at one cpu, you will still only get
> affinity for that one cpu.  You may want to consider changing the hintpolicy for
> those interrupts to ignore, to ensure that you have affinity for two cpus.

Hi Neil,

I changed the hint policy to ignore for these IRQs but still I observe
only one CPU
is busy with interrupt processing and eventually I am observe softlockups.

Thanks,
Sreekanth

>
> Beyond that though, the kernel is in control of irq delivery.  Normally the
> configured hardware delivery policy is to select the highest priority cpu that
> isn't already servicing an interrupt (to maximize cache hit rates).  If the irq
> rate is sufficiently slow however, it will always hit the same cpu, because it
> isn't blocked by another interrupt.
>
> Best
> Neil
>



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