Messages in this thread | | | Date | Thu, 30 Sep 2021 11:53:48 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH 4/5] irq_work: Handle some irq_work in SOFTIRQ on PREEMPT_RT |
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On 2021-09-30 11:07:18 [+0200], Peter Zijlstra wrote: > > IIRC we have existing problems in -RT due to this irq_work softirq muck.
We have existing problems in -RT due irq_work being used without knowing the consequences.
> I think the problem was something Jolsa found a while ago, where perf > defers to an irq_work (from NMI context) and that irq_work wants to > deliver signals, which it can't on -RT, so the whole thing gets punted > to softirq. With the end-result that if you self-profile RT tasks, > things come apart or something.
For signals (at least on x86) we this ARCH_RT_DELAYS_SIGNAL_SEND thingy where the signal is delayed until exit_to_user_mode_loop().
perf_pending_event() is the only non-HARD on RT (on the perf side). I think that is due to perf_event_wakeup() where we have wake_up_all() and read_lock_irqsave().
> There might have been others as well, I don't know. But generally I > think we want *less* softirq, not more.
I agree. The anonymous softirqs concept brings problems of its own. But what should I do with things like that: - kernel/trace/ring_buffer.c rb_wake_up_waiters() kernel/bpf/ringbuf.c bpf_ringbuf_notify() wake_up_all()
- drivers/acpi/apei/ghes.c ghes_proc_in_irq() spinlock_t
- drivers/dma-buf/dma-fence-array.c irq_dma_fence_array_work() spinlock_t, callbacks, potential kfree().
- drivers/gpu/drm/i915/gt/intel_breadcrumbs.c signal_irq_work() spinlock_t, rpm_put() -> wake_up_var(), callbacks, kref_put() like constructs which may free memory.
I didn't look at _all_ of them but just a few. And the only one I looked at and didn't add to the list was drivers/edac/igen6_edac.c ecclog_irq_work_cb()
which simply reads PCI registers (which acquires raw_spinlock_t only (however only on x86 are those raw_spinlock_t now that looked around)) and does schedule_work(). All harmless.
I *think* the irq_work in printk is going to leave once John is done with it. But there are way more of these things in the kernel now compared to when I first pushed them into softirq because they were causing trouble.
Sebastian
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