(While evaluating some changes to the page allocator) I ran into an
issue with ksoftirqd getting too much CPU sched time.

I bisected the problem to
 a499a5a14dbd ("sched/cputime: Increment kcpustat directly on irqtime account")

 a499a5a14dbd1d0315a96fc62a8798059325e9e6 is the first bad commit
 commit a499a5a14dbd1d0315a96fc62a8798059325e9e6
 Author: Frederic Weisbecker [off-list ref]
 Date:   Tue Jan 31 04:09:32 2017 +0100

    sched/cputime: Increment kcpustat directly on irqtime account
    
    The irqtime is accounted is nsecs and stored in
    cpu_irq_time.hardirq_time and cpu_irq_time.softirq_time. Once the
    accumulated amount reaches a new jiffy, this one gets accounted to the
    kcpustat.
    
    This was necessary when kcpustat was stored in cputime_t, which could at
    worst have jiffies granularity. But now kcpustat is stored in nsecs
    so this whole discretization game with temporary irqtime storage has
    become unnecessary.
    
    We can now directly account the irqtime to the kcpustat.
    
    Signed-off-by: Frederic Weisbecker [off-list ref]
    Cc: Benjamin Herrenschmidt [off-list ref]
    Cc: Fenghua Yu [off-list ref]
    Cc: Heiko Carstens [off-list ref]
    Cc: Linus Torvalds [off-list ref]
    Cc: Martin Schwidefsky [off-list ref]
    Cc: Michael Ellerman [off-list ref]
    Cc: Paul Mackerras [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Rik van Riel [off-list ref]
    Cc: Stanislaw Gruszka [off-list ref]
    Cc: Thomas Gleixner [off-list ref]
    Cc: Tony Luck [off-list ref]
    Cc: Wanpeng Li [off-list ref]
    Link: http://lkml.kernel.org/r/1485832191-26889-17-git-send-email-fweisbec@gmail.com
    Signed-off-by: Ingo Molnar [off-list ref]

The reproducer is running a userspace udp_sink[1] program, and taskset
pinning the process to the same CPU as softirq RX is running on, and
starting a UDP flood with pktgen (tool part of kernel tree:
samples/pktgen/pktgen_sample03_burst_single_flow.sh).

[1] udp_sink
 https://github.com/netoptimizer/network-testing/blob/master/src/udp_sink.c

The expected results (after commit 4cd13c21b207 ("softirq: Let
ksoftirqd do its job")) is that the scheduler split the CPU time 50/50
between udp_sink and ksoftirqd.

After this commit, the udp_sink program does not get any sched CPU
time, and no packets are delivered to userspace.  (All packets are
dropped by softirq due to a full socket queue, nstat UdpRcvbufErrors).

A related symptom is that ksoftirqd no longer get accounted in top.

$ grep CONFIG_IRQ_TIME_ACCOUNTING .config
CONFIG_IRQ_TIME_ACCOUNTING=y

Full .config uploaded here[2]:
 [2] http://people.netfilter.org/hawk/kconfig/config02-bisect-softirq-a499a5a14dbd

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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