netdev - Re: Bad performance on modified pktgen in 4.0 vs 3.17 kernel.

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Date:	Thu, 07 May 2015 21:11:50 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	netdev <netdev@...r.kernel.org>
Subject: Re: Bad performance on modified pktgen in 4.0 vs 3.17 kernel.

My problem was self inflicted:  I had lockdep and related things enabled.

Runs fine w/out that extra debug in there.

THanks,
Ben


On 04/29/2015 04:39 PM, Ben Greear wrote:
> We run a hacked version of pktgen, it has some pkt-rx logic, and probably spends more time
> grabbing timestamps than stock code.  It also should not be doing any busy-spins for sleeping.
>
> You can see pktgen changes, supporting patches, and various other stuff here:
>
> http://dmz2.candelatech.com/git/gitweb.cgi?p=linux-4.0.dev.y/.git;a=summary
> git clone git://dmz2.candelatech.com/linux-4.0.dev.y
>
>
> On a 64-bit atom system, with e1000 driver, we see around 50% cpu usage
> when running 40,000 pkts per second on two interfaces on the 3.17.8+ kernel.
>
> # cat perf-top-3-17.txt
>     PerfTop:    3682 irqs/sec  kernel:78.7%  exact:  0.0% [4000Hz cycles],  (all, 4 CPUs)
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>       3.43%  [kernel]       [k] pktgen_thread_worker
>       2.47%  libc-2.20.so   [.] __strstr_sse2
>       2.31%  [kernel]       [k] e1000_xmit_frame
>       2.25%  [kernel]       [k] number.isra.1
>       2.18%  [kernel]       [k] vsnprintf
>       1.96%  libc-2.20.so   [.] __GI___strcmp_ssse3
>       1.84%  [kernel]       [k] format_decode
>       1.80%  [kernel]       [k] build_skb
>       1.79%  [kernel]       [k] kallsyms_expand_symbol.constprop.1
>       1.76%  [kernel]       [k] native_read_tsc
>       1.74%  perf           [.] rb_next
>       1.57%  [kernel]       [k] getRelativeCurNs
>       1.48%  perf           [.] symbols__insert
>       1.10%  perf           [.] hex2u64
>       1.07%  [kernel]       [k] e1000_irq_enable
>       1.06%  [kernel]       [k] timekeeping_get_ns
>       1.03%  [kernel]       [k] e1000_clean_rx_irq
>       1.00%  [kernel]       [k] __getnstimeofday64
>       0.97%  [kernel]       [k] string.isra.6
>       0.97%  [kernel]       [k] do_raw_spin_lock
>       0.97%  [kernel]       [k] kmem_cache_alloc
>       0.94%  [kernel]       [k] e1000_intr_msi
>
>
> On 4.0, there is significantly more CPU usage.  I tried copying the pktgen.c from 3.17 to 4.0
> and that did not have any noticeable affect, so I think it must be something outside of my changes.
>
> # cat perf-top-40.txt
>     PerfTop:    4566 irqs/sec  kernel:87.4%  exact:  0.0% [4000Hz cycles],  (all, 4 CPUs)
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>      20.72%  [kernel]       [k] mwait_idle_with_hints.constprop.2
>      10.98%  [kernel]       [k] __lock_acquire
>       3.30%  [kernel]       [k] pktgen_thread_worker
>       2.41%  [kernel]       [k] arch_local_save_flags
>       2.25%  [kernel]       [k] e1000_xmit_frame
>       1.83%  [kernel]       [k] lock_release
>       1.57%  [kernel]       [k] lock_acquire
>       1.54%  [kernel]       [k] trace_hardirqs_on_caller
>       1.50%  libc-2.20.so   [.] __strstr_sse2
>       1.41%  [kernel]       [k] number.isra.1
>       1.22%  [kernel]       [k] trace_hardirqs_off_caller
>       1.20%  [kernel]       [k] kallsyms_expand_symbol.constprop.1
>       1.19%  [kernel]       [k] build_skb
>       1.18%  [kernel]       [k] format_decode
>       1.17%  [kernel]       [k] hlock_class
>       1.17%  [kernel]       [k] arch_local_irq_restore
>       1.09%  [kernel]       [k] vsnprintf
>       1.00%  [kernel]       [k] arch_local_irq_save
>       0.97%  libc-2.20.so   [.] __GI___strcmp_ssse3
>       0.97%  [kernel]       [k] mark_held_locks
>       0.89%  [kernel]       [k] mark_lock
>
>
> We see similar jump in CPU usage in the 4.0 kernel when using the 40G Intel NIC/driver
> on an E5 system, so it is probably not just something to do with the driver.
>
> Due to hooks in the pkt rx logic (and changes to the stock kernel code in that area between
> 3.17 and 4.), this will not be trivial to do an automated bisect, so I'm hoping to not
> have to do that...
>
> I'm curious if anyone has seen any similar performance degradation, and whether there
> are any ideas what might be the problem.
>
> Thanks,
> Ben
>
>
>

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com
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