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

Frederic Weisbecker
2017-03-28T21:11:48Z

On Tue, Mar 28, 2017 at 05:23:03PM +0200, Jesper Dangaard Brouer wrote:
> On Tue, 28 Mar 2017 16:34:36 +0200
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
> > 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?
>
> No, you misunderstood. I run pktgen on another physical machine, which
> is sending UDP packets towards my Device-Under-Test (DUT) target. The
> DUT-target is receiving packets and I observe which CPU the NIC is
> delivering these packets to.

Ah ok, so I tried to run pktgen on another machine and I get that strange write error:

# ./pktgen_sample03_burst_single_flow.sh -d 192.168.1.3 -i wlan0
./functions.sh: ligne 76 : echo: erreur d'�criture : Erreur inconnue 524
ERROR: Write error(1) occurred cmd: "clone_skb 100000 > /proc/net/pktgen/wlan0@0"

Any idea?

>
> E.g determine RX-CPU via mpstat command:
> mpstat -P ALL -u -I SCPU -I SUM 2
>
> I then start udp_sink, pinned to the RX-CPU, like:
> sudo taskset -c 2 ./udp_sink --port 9 --count $((10**6)) --recvmsg --repeat 1000

Ah thanks for these hints!

> > > 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!
>
> Good to hear you can reproduce it! :-)

Well, since I was generating the packets locally, maybe it didn't trigger
the expected interrupts...