Messages in this thread Patch in this message | | | From | Paolo Abeni <> | Subject | [PATCH] revert: "softirq: Let ksoftirqd do its job" | Date | Mon, 8 May 2023 08:17:44 +0200 |
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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 <pabeni@redhat.com> Tested-by: Jason Xing <kerneljasonxing@gmail.com> --- kernel/softirq.c | 22 ++-------------------- 1 file changed, 2 insertions(+), 20 deletions(-)
diff --git a/kernel/softirq.c b/kernel/softirq.c index 1b725510dd0f..807b34ccd797 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); -- 2.40.0
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