Messages in this thread Patch in this message | | | Subject | Re: regression with napi/softirq ? | From | Eric Dumazet <> | Date | Thu, 18 Jul 2019 08:58:14 +0200 |
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On 7/17/19 11:52 PM, Thomas Gleixner wrote: > Sudip, > > On Wed, 17 Jul 2019, Sudip Mukherjee wrote: >> On Wed, Jul 17, 2019 at 9:53 PM Thomas Gleixner <tglx@linutronix.de> wrote: >>> You can hack ksoftirq_running() to return always false to avoid this, but >>> that might cause application starvation and a huge packet buffer backlog >>> when the amount of incoming packets makes the CPU do nothing else than >>> softirq processing. >> >> I tried that now, it is better but still not as good as v3.8 >> Now I am getting 375.9usec as the maximum time between raising the softirq >> and it starting to execute and packet drops still there. >> >> And just a thought, do you think there should be a CONFIG_ option for >> this feature of ksoftirqd_running() so that it can be disabled if needed >> by users like us? > > If at all then a sysctl to allow runtime control. > >> Can you please think of anything else that might have changed which I still need >> to change to make the time comparable to v3.8.. > > Something with in that small range of: > > 63592 files changed, 13783320 insertions(+), 5155492 deletions(-) > > :) > > Seriously, that can be anything. > > Can you please test with Linus' head of tree and add some more > instrumentation, so we can see what holds off softirqs from being > processed. If the ksoftirqd enforcement is disabled, then the only reason > can be a long lasting softirq disabled region. Tracing should tell.
ksoftirqd might be spuriously scheduled from tx path, when __qdisc_run() also reacts to need_resched().
By raising NET_TX while we are processing NET_RX (say we send a TCP ACK packet in response to incoming packet), we force __do_softirq() to perform another loop, but before doing an other round, it will also check need_resched() and eventually call wakeup_softirqd()
I wonder if following patch makes any difference.
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 11c03cf4aa74b44663c74e0e3284140b0c75d9c4..ab736e974396394ae6ba409868aaea56a50ad57b 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -377,6 +377,8 @@ void __qdisc_run(struct Qdisc *q) int packets; while (qdisc_restart(q, &packets)) { + if (qdisc_is_empty(q)) + break; /* * Ordered by possible occurrence: Postpone processing if * 1. we've exceeded packet quota
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