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84 lines
2.6 KiB
Diff
84 lines
2.6 KiB
Diff
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From: Eric Dumazet <edumazet@google.com>
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Date: Wed, 31 Aug 2016 10:42:29 -0700
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Subject: [PATCH] softirq: let ksoftirqd do its job
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A while back, Paolo and Hannes sent an RFC patch adding threaded-able
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napi poll loop support : (https://patchwork.ozlabs.org/patch/620657/)
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The problem seems to be that softirqs are very aggressive and are often
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handled by the current process, even if we are under stress and that
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ksoftirqd was scheduled, so that innocent threads would have more chance
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to make progress.
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This patch makes sure that if ksoftirq is running, we let it
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perform the softirq work.
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Jonathan Corbet summarized the issue in https://lwn.net/Articles/687617/
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Tested:
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- NIC receiving traffic handled by CPU 0
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- UDP receiver running on CPU 0, using a single UDP socket.
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- Incoming flood of UDP packets targeting the UDP socket.
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Before the patch, the UDP receiver could almost never get cpu cycles and
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could only receive ~2,000 packets per second.
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After the patch, cpu cycles are split 50/50 between user application and
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ksoftirqd/0, and we can effectively read ~900,000 packets per second,
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a huge improvement in DOS situation. (Note that more packets are now
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dropped by the NIC itself, since the BH handlers get less cpu cycles to
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drain RX ring buffer)
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Since the load runs in well identified threads context, an admin can
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more easily tune process scheduling parameters if needed.
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Reported-by: Paolo Abeni <pabeni@redhat.com>
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Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Cc: David Miller <davem@davemloft.net
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Cc: Jesper Dangaard Brouer <jbrouer@redhat.com>
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Cc: Peter Zijlstra <peterz@infradead.org>
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Cc: Rik van Riel <riel@redhat.com>
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---
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--- a/kernel/softirq.c
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+++ b/kernel/softirq.c
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@@ -78,6 +78,17 @@ static void wakeup_softirqd(void)
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}
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/*
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+ * If ksoftirqd is scheduled, we do not want to process pending softirqs
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+ * right now. Let ksoftirqd handle this at its own rate, to get fairness.
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+ */
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+static bool ksoftirqd_running(void)
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+{
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+ struct task_struct *tsk = __this_cpu_read(ksoftirqd);
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+
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+ return tsk && (tsk->state == TASK_RUNNING);
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+}
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+
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+/*
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* preempt_count and SOFTIRQ_OFFSET usage:
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* - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
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* softirq processing.
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@@ -313,7 +324,7 @@ asmlinkage __visible void do_softirq(voi
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pending = local_softirq_pending();
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- if (pending)
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+ if (pending && !ksoftirqd_running())
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do_softirq_own_stack();
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local_irq_restore(flags);
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@@ -340,6 +351,9 @@ void irq_enter(void)
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static inline void invoke_softirq(void)
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{
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+ if (ksoftirqd_running())
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+ return;
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+
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if (!force_irqthreads) {
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#ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
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/*
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