netdev - Re: regression with napi/softirq ?

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Date:   Thu, 18 Jul 2019 17:08:20 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression with napi/softirq ?



On 7/18/19 2:55 PM, Sudip Mukherjee wrote:

> Thanks Eric. But there is no improvement in delay between
> softirq_raise and softirq_entry with this change.
> But moving to a later kernel (linus master branch? ) like Thomas has
> said in the other mail might be difficult atm. I can definitely
> move to v4.14.133 if that helps. Thomas ?

If you are tracking max latency then I guess you have to tweak SOFTIRQ_NOW_MASK
to include NET_RX_SOFTIRQ

The patch I gave earlier would only lower the probability of events, not completely get rid of them.



diff --git a/kernel/softirq.c b/kernel/softirq.c
index 0427a86743a46b7e1891f7b6c1ff585a8a1695f5..302046dd8d7e6740e466c422954f22565fe19e69 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -81,7 +81,7 @@ static void wakeup_softirqd(void)
  * right now. Let ksoftirqd handle this at its own rate, to get fairness,
  * unless we're doing some of the synchronous softirqs.
  */
-#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
+#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ) | (1 << NET_RX_SOFTIRQ))
 static bool ksoftirqd_running(unsigned long pending)
 {
        struct task_struct *tsk = __this_cpu_read(ksoftirqd);