Re: [PATCH] softirq: let ksoftirqd do its job
From: Jesper Dangaard Brouer <hidden>
Date: 2016-08-31 19:40:56
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On Wed, 31 Aug 2016 10:42:29 -0700 Eric Dumazet [off-list ref] wrote:
From: Eric Dumazet <edumazet@google.com> A while back, Paolo and Hannes sent an RFC patch adding threaded-able napi poll loop support : (https://patchwork.ozlabs.org/patch/620657/) The problem seems to be that softirqs are very aggressive and are often handled by the current process, even if we are under stress and that ksoftirqd was scheduled, so that innocent threads would have more chance to make progress. This patch makes sure that if ksoftirq is running, we let it perform the softirq work. Jonathan Corbet summarized the issue in https://lwn.net/Articles/687617/ Tested: - NIC receiving traffic handled by CPU 0 - UDP receiver running on CPU 0, using a single UDP socket. - Incoming flood of UDP packets targeting the UDP socket. Before the patch, the UDP receiver could almost never get cpu cycles and could only receive ~2,000 packets per second. After the patch, cpu cycles are split 50/50 between user application and ksoftirqd/0, and we can effectively read ~900,000 packets per second, a huge improvement in DOS situation. (Note that more packets are now dropped by the NIC itself, since the BH handlers get less cpu cycles to drain RX ring buffer)
I can confirm the improvement of approx 900Kpps (no wonder people have
been complaining about DoS against UDP/DNS servers).
BUT during my extensive testing, of this patch, I also think that we
have not gotten to the bottom of this. I was expecting to see a higher
(collective) PPS number as I add more UDP servers, but I don't.
Running many UDP netperf's with command:
super_netperf 4 -H 198.18.50.3 -l 120 -t UDP_STREAM -T 0,0 -- -m 1472 -n -N
With 'top' I can see ksoftirq are still getting a higher %CPU time:
PID %CPU TIME+ COMMAND
3 36.5 2:28.98 ksoftirqd/0
10724 9.6 0:01.05 netserver
10722 9.3 0:01.05 netserver
10723 9.3 0:01.05 netserver
10725 9.3 0:01.05 netserver
Since the load runs in well identified threads context, an admin can more easily tune process scheduling parameters if needed.
With this patch applied, I found that changing the UDP server process,
scheduler policy to SCHED_RR or SCHED_FIFO gave me a performance boost
from 900Kpps to 1.7Mpps, and not a single UDP packet dropped (even with
a single UDP stream, also tested with more)
Command used:
sudo chrt --rr -p 20 $(pgrep netserver)
The scheduling picture also change a lot:
PID %CPU TIME+ COMMAND
10783 24.3 0:21.53 netserver
10784 24.3 0:21.53 netserver
10785 24.3 0:21.52 netserver
10786 24.3 0:21.50 netserver
3 2.7 3:12.18 ksoftirqd/0
quoted hunk ↗ jump to hunk
Reported-by: Paolo Abeni <pabeni@redhat.com> Reported-by: Hannes Frederic Sowa <redacted> Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: David Miller <davem@davemloft.net Cc: Jesper Dangaard Brouer <redacted> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Rik van Riel <redacted> --- kernel/softirq.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)diff --git a/kernel/softirq.c b/kernel/softirq.c index 17caf4b63342..8ed90e3a88d6 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c@@ -78,6 +78,17 @@ static void wakeup_softirqd(void) } /* + * If ksoftirqd is scheduled, we do not want to process pending softirqs + * right now. Let ksoftirqd handle this at its own rate, to get fairness. + */ +static bool ksoftirqd_running(void) +{ + struct task_struct *tsk = __this_cpu_read(ksoftirqd); + + return tsk && (tsk->state == TASK_RUNNING); +} + +/* * preempt_count and SOFTIRQ_OFFSET usage: * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving * softirq processing.@@ -313,7 +324,7 @@ asmlinkage __visible void do_softirq(void) pending = local_softirq_pending(); - if (pending) + if (pending && !ksoftirqd_running()) do_softirq_own_stack(); local_irq_restore(flags);@@ -340,6 +351,9 @@ void irq_enter(void) static inline void invoke_softirq(void) { + if (ksoftirqd_running()) + return; + if (!force_irqthreads) { #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK /*
-- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer