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

Wanpeng Li
2017-03-28T10:34:57Z

2017-03-28 16:14 GMT+08:00 Jesper Dangaard Brouer <brouer@redhat.com>:
>
> (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).
>
> [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

void irqtime_account_irq(struct task_struct *curr)
{
struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
u64 *cpustat = kcpustat_this_cpu->cpustat;
s64 delta;
int cpu;

if (!sched_clock_irqtime)
return;

cpu = smp_processor_id();
delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;

sched_clock_cpu(cpu) should be converted from cputime to ns. However,
cputime_to_nsecs() API is removed by
https://lkml.org/lkml/2017/1/22/230 for generic usage, so could you
try the below patch just for testing?

diff --git a/include/linux/sched/cputime.h b/include/linux/sched/cputime.h
index 4c5b973..166efba 100644
--- a/include/linux/sched/cputime.h
+++ b/include/linux/sched/cputime.h
@@ -7,14 +7,14 @@
* cputime accounting APIs:
*/

-#ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
-#include <asm/cputime.h>
+#define cputime_div(__ct, divisor) div_u64((__force u64)__ct, divisor)
+#define cputime_to_usecs(__ct) \
+ cputime_div(__ct, NSEC_PER_USEC)

#ifndef cputime_to_nsecs
# define cputime_to_nsecs(__ct) \
(cputime_to_usecs(__ct) * NSEC_PER_USEC)
#endif
-#endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */

#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
extern void task_cputime(struct task_struct *t,
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index f3778e2b..68064d1 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -49,7 +49,7 @@ void irqtime_account_irq(struct task_struct *curr)
return;

cpu = smp_processor_id();
- delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;
+ delta = cputime_to_nsecs(sched_clock_cpu(cpu)) - irqtime->irq_start_time;
irqtime->irq_start_time += delta;

u64_stats_update_begin(&irqtime->sync);

Regards,
Wanpeng Li