[lkml]   [2017]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Bisected softirq accounting issue in v4.11-rc1~170^2~28
On Tue, Mar 28, 2017 at 10:14:03AM +0200, Jesper Dangaard Brouer wrote:
> (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 <>
> 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 <>
> Cc: Benjamin Herrenschmidt <>
> Cc: Fenghua Yu <>
> Cc: Heiko Carstens <>
> Cc: Linus Torvalds <>
> Cc: Martin Schwidefsky <>
> Cc: Michael Ellerman <>
> Cc: Paul Mackerras <>
> Cc: Peter Zijlstra <>
> Cc: Rik van Riel <>
> Cc: Stanislaw Gruszka <>
> Cc: Thomas Gleixner <>
> Cc: Tony Luck <>
> Cc: Wanpeng Li <>
> Link:
> Signed-off-by: Ingo Molnar <>
> 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/

So that means I need to run udp_sink on the same CPU than pktgen?

> [1] udp_sink
> 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.

I guess you mean that this is what happened before this commit?

> 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.

That's indeed what I observe. udp_sink has almost no CPU time, neither has
ksoftirqd but kpktgend_0 has everything.

Finally a bug I can reproduce!

 \ /
  Last update: 2017-03-28 16:37    [W:0.091 / U:0.592 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site