Messages in this thread | | | Date | Tue, 8 Feb 2022 23:57:05 -0800 | From | Christoph Hellwig <> | Subject | Re: [PATCH v3] smp: Wake ksoftirqd on PREEMPT_RT instead do_softirq(). |
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On Tue, Feb 08, 2022 at 05:54:07PM +0100, Sebastian Andrzej Siewior wrote: > The softirq implementation on PREEMPT_RT does not provide do_softirq(). The > softirq can not be handled directly here because migration_cpu_stop() is > invoked with disabled preemption/ interrupts. > > A known user of scheduling softirqs from a remote function call is the block > layer. It won't happen on PREEMPT_RT because it doesn't make sense for > latency/ performance reasons and is disabled. Nevertheless this should > be handled in case of a new user pops up rather than simply ignoring it. > > Waking ksoftirqd unconditionally can be problematic if softirqs were already > pending but not yet handled. This can happen since the migration thread > is running at a high priority and able to preempt a threaded-interrupt. > The woken-up ksoftirqd would catch-up all pending (and later raised) > softirqs which is not desired on PREEMPT_RT since it is no longer > handled where it has been originally raised. This in turn delays the > actual processing until a SCHED_OTHER task can run. > > Wake the softirq thread on PREEMPT_RT if a remote function call raised > softirqs. Add warning in this case since this condition is not desired. > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > v2…v3: > - Only wake ksoftirqd if the softirqs were raised wthin > flush_smp_call_function_queue(). > - Add a warning in the wake case. > v1…v2: Drop an empty line. > > kernel/smp.c | 21 ++++++++++++++++++--- > 1 file changed, 18 insertions(+), 3 deletions(-) > --- > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -691,10 +691,25 @@ void flush_smp_call_function_from_idle(v > cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU, > smp_processor_id(), CFD_SEQ_IDLE); > local_irq_save(flags); > - flush_smp_call_function_queue(true); > - if (local_softirq_pending()) > - do_softirq(); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
I still absolutely hate these pointless negations in simple if statements.
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