Re: [PATCH AUTOSEL 5.10 016/137] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT
From: Pavel Machek <hidden>
Date: 2021-07-12 21:53:04
Also in:
lkml, stable
Hi!
__napi_schedule_irqoff() is an optimized version of __napi_schedule() which can be used where it is known that interrupts are disabled, e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer callbacks. On PREEMPT_RT enabled kernels this assumptions is not true. Force- threaded interrupt handlers and spinlocks are not disabling interrupts and the NAPI hrtimer callback is forced into softirq context which runs with interrupts enabled as well. Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole game so make __napi_schedule_irqoff() invoke __napi_schedule() for PREEMPT_RT kernels. The callers of ____napi_schedule() in the networking core have been audited and are correct on PREEMPT_RT kernels as well.
I see this is queued to kernels as old as 4.4... Is it good idea? PREEMPT_RT is not usable there without extra patches, so it does not really fix anything user visible.... Best regards, Pavel
quoted hunk ↗ jump to hunk
index 0c9ce36afc8c..2fdf30eefc59 100644--- a/net/core/dev.c +++ b/net/core/dev.c@@ -6433,11 +6433,18 @@ EXPORT_SYMBOL(napi_schedule_prep); * __napi_schedule_irqoff - schedule for receive * @n: entry to schedule * - * Variant of __napi_schedule() assuming hard irqs are masked + * Variant of __napi_schedule() assuming hard irqs are masked. + * + * On PREEMPT_RT enabled kernels this maps to __napi_schedule() + * because the interrupt disabled assumption might not be true + * due to force-threaded interrupts and spinlock substitution. */ void __napi_schedule_irqoff(struct napi_struct *n) { - ____napi_schedule(this_cpu_ptr(&softnet_data), n); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + ____napi_schedule(this_cpu_ptr(&softnet_data), n); + else + __napi_schedule(n); } EXPORT_SYMBOL(__napi_schedule_irqoff);
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