From 79a793effc8b0bc24351f9109b369f20e5fbbe66 Mon Sep 17 00:00:00 2001 From: Paolo Abeni Date: Mon, 8 May 2023 08:17:44 +0200 Subject: [PATCH 55/64] revert: "softirq: Let ksoftirqd do its job" Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.1/older/patches-6.1.82-rt27.tar.xz Due to the mentioned commit, when the ksoftirqd processes take charge of softirq processing, the system can experience high latencies. In the past a few workarounds have been implemented for specific side-effects of the above: commit 1ff688209e2e ("watchdog: core: make sure the watchdog_worker is not deferred") commit 8d5755b3f77b ("watchdog: softdog: fire watchdog even if softirqs do not get to run") commit 217f69743681 ("net: busy-poll: allow preemption in sk_busy_loop()") commit 3c53776e29f8 ("Mark HI and TASKLET softirq synchronous") but the latency problem still exists in real-life workloads, see the link below. The reverted commit intended to solve a live-lock scenario that can now be addressed with the NAPI threaded mode, introduced with commit 29863d41bb6e ("net: implement threaded-able napi poll loop support"), and nowadays in a pretty stable status. While a complete solution to put softirq processing under nice resource control would be preferable, that has proven to be a very hard task. In the short term, remove the main pain point, and also simplify a bit the current softirq implementation. Note that this change also reverts commit 3c53776e29f8 ("Mark HI and TASKLET softirq synchronous") and commit 1342d8080f61 ("softirq: Don't skip softirq execution when softirq thread is parking"), which are direct follow-ups of the feature commit. A single change is preferred to avoid known bad intermediate states introduced by a patch series reverting them individually. Link: https://lore.kernel.org/netdev/305d7742212cbe98621b16be782b0562f1012cb6.camel@redhat.com/ Signed-off-by: Paolo Abeni Tested-by: Jason Xing Reviewed-by: Jakub Kicinski Reviewed-by: Eric Dumazet Reviewed-by: Sebastian Andrzej Siewior Link: https://lore.kernel.org/r/57e66b364f1b6f09c9bc0316742c3b14f4ce83bd.1683526542.git.pabeni@redhat.com Signed-off-by: Sebastian Andrzej Siewior (cherry picked from commit b8a04a538ed4755dc97c403ee3b8dd882955c98c) Signed-off-by: Clark Williams --- kernel/softirq.c | 22 ++-------------------- 1 file changed, 2 insertions(+), 20 deletions(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 82f3e68fbe22..af9e879bbbf7 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -80,21 +80,6 @@ static void wakeup_softirqd(void) wake_up_process(tsk); } -/* - * 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, - * unless we're doing some of the synchronous softirqs. - */ -#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ)) -static bool ksoftirqd_running(unsigned long pending) -{ - struct task_struct *tsk = __this_cpu_read(ksoftirqd); - - if (pending & SOFTIRQ_NOW_MASK) - return false; - return tsk && task_is_running(tsk) && !__kthread_should_park(tsk); -} - #ifdef CONFIG_TRACE_IRQFLAGS DEFINE_PER_CPU(int, hardirqs_enabled); DEFINE_PER_CPU(int, hardirq_context); @@ -236,7 +221,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt) goto out; pending = local_softirq_pending(); - if (!pending || ksoftirqd_running(pending)) + if (!pending) goto out; /* @@ -432,9 +417,6 @@ static inline bool should_wake_ksoftirqd(void) static inline void invoke_softirq(void) { - if (ksoftirqd_running(local_softirq_pending())) - return; - if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) { #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK /* @@ -468,7 +450,7 @@ asmlinkage __visible void do_softirq(void) pending = local_softirq_pending(); - if (pending && !ksoftirqd_running(pending)) + if (pending) do_softirq_own_stack(); local_irq_restore(flags); -- 2.44.0