Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Kehuan Feng <kehuan.feng@gmail.com>]

Hi Hillf,

> Let’s see if TCQ_F_NOLOC is making fq_codel different in your testing.

I assume you meant disabling NOLOCK for pfifo_fast.

Here is the modification,

--- ./net/sched/sch_generic.c.orig      2020-08-24 22:02:04.589830751 +0800
+++ ./net/sched/sch_generic.c   2020-08-27 10:17:10.148977195 +0800
@@ -792,7 +792,7 @@
        .dump           =       pfifo_fast_dump,
        .change_tx_queue_len =  pfifo_fast_change_tx_queue_len,
        .owner          =       THIS_MODULE,
-       .static_flags   =       TCQ_F_NOLOCK | TCQ_F_CPUSTATS,
+       .static_flags   =       TCQ_F_CPUSTATS,

The issue never happen again with it for over 3 hours stressing. And I
restarted the test for two times. No any surprising. Quite stable...


Hillf Danton <hdanton@sina.com> 于2020年8月26日周三 下午2:37写道：
>
> Hi Feng,
>
>
>
> On Wed, 26 Aug 2020 11:12:38 +0800 Fengkehuan Feng wrote:
>
> >Hi Hillf,
> >
> >I just gave more tries on the patch, and it seems not that good as what I told in last email.
>
> >I could see more packets getting stuck now...
>
> We have more to learn here:P
>
> >
> >Let me explain what I am facing in detail in case we are not aligning to fix the same problem.
> >
> >Our application is in deep learning scenario and it's based on NVIDIA NCCL to do
>
> >collective communication intra-node or inter-node (to be more specific, it's data
>
> > all-reduce on two servers witch 8 GPU nodes each).
> >NCCL can support data transmission through TCP/RDMA/GDR. In normal, it takes
>
> > about 1000 us for TCP or less for RDMA/GDR to transmit 512KB packet, but
>
> > sometimes it tooks hundreds of millisecond or several seconds to get completed.
> >
>
> >When we change the default qdisc from pfifo_fast to fq_codel, the issue never
>
> > happen, so we suspect it's something wrong within the networking stack (but
>
> > it's a bit strange that RDMA or GDR has the same problem)
>
> Let’s see if TCQ_F_NOLOC is making fq_codel different in your testing.
>
>
>
> --- a/net/sched/sch_generic.c
>
> +++ b/net/sched/sch_generic.c
>
> @@ -791,7 +791,7 @@ struct Qdisc_ops pfifo_fast_ops __read_m
>
>       .dump           =    pfifo_fast_dump,
>
>       .change_tx_queue_len =  pfifo_fast_change_tx_queue_len,
>
>       .owner          =    THIS_MODULE,
>
> -     .static_flags   =    TCQ_F_NOLOCK | TCQ_F_CPUSTATS,
>
> +    .static_flags   =    TCQ_F_CPUSTATS,
>
> };
>
> EXPORT_SYMBOL(pfifo_fast_ops);
>
> --
>
>
>
> >
> >Here is the log print from our test application,
> >
> >size: 512KB, use_time: 1118us, speed: 0.436745GB/s
> >size: 512KB, use_time: 912us, speed: 0.535396GB/s
> >size: 512KB, use_time: 1023us, speed: 0.477303GB/s
> >size: 512KB, use_time: 919us, speed: 0.531318GB/s
> >size: 512KB, use_time: 1129us, speed: 0.432490GB/s
> >size: 512KB, use_time: 2098748us, speed: 0.000233GB/s
> >size: 512KB, use_time: 1018us, speed: 0.479648GB/s
> >size: 512KB, use_time: 1120us, speed: 0.435965GB/s
> >size: 512KB, use_time: 1071us, speed: 0.455912GB/
>
>
>
>
>
> JFYI I failed to find this message at lore.kernel.org perhaps
>
> because of pure text mail.
>
>
>
> Thanks
>
> Hillf