Re: Bisected softirq accounting issue in v4.11-rc1~170^2~28

Frederic Weisbecker
2017-03-28T14:35:02Z

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 <fweisbec@gmail.com>
> 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 <fweisbec@gmail.com>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Stanislaw Gruszka <sgruszka@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> Link: http://lkml.kernel.org/r/1485832191-26889-17-git-send-email-fweisbec@gmail.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>
> 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).

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

>
> [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.

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!