Messages in this thread Patch in this message | | | Date | Mon, 7 Dec 2020 01:23:43 +0100 | From | Frederic Weisbecker <> | Subject | Re: [patch V2 2/9] irqtime: Make accounting correct on RT |
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On Fri, Dec 04, 2020 at 06:01:53PM +0100, Thomas Gleixner wrote: > vtime_account_irq and irqtime_account_irq() base checks on preempt_count() > which fails on RT because preempt_count() does not contain the softirq > accounting which is seperate on RT. > > These checks do not need the full preempt count as they only operate on the > hard and softirq sections. > > Use irq_count() instead which provides the correct value on both RT and non > RT kernels. The compiler is clever enough to fold the masking for !RT: > > 99b: 65 8b 05 00 00 00 00 mov %gs:0x0(%rip),%eax > - 9a2: 25 ff ff ff 7f and $0x7fffffff,%eax > + 9a2: 25 00 ff ff 00 and $0xffff00,%eax > > Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > --- > V2: New patch > --- > kernel/sched/cputime.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > --- a/kernel/sched/cputime.c > +++ b/kernel/sched/cputime.c > @@ -60,7 +60,7 @@ void irqtime_account_irq(struct task_str > cpu = smp_processor_id(); > delta = sched_clock_cpu(cpu) - irqtime->irq_start_time; > irqtime->irq_start_time += delta; > - pc = preempt_count() - offset; > + pc = irq_count() - offset;
There are many preempt_count() users all around waiting for similar issues. Wouldn't it be more reasonable to have current->softirq_disable_cnt just saving the softirq count on context switch?
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d2003a7d5ab5..6c899c35d6ba 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3469,6 +3469,10 @@ static inline void prepare_task(struct task_struct *next) static inline void finish_task(struct task_struct *prev) { +#ifdef CONFIG_PREEMPT_RT + prev->softirq_disable_cnt = softirq_count(); + __preempt_count_sub(prev->softirq_disable_cnt); +#endif #ifdef CONFIG_SMP /* * This must be the very last reference to @prev from this CPU. After @@ -3610,6 +3614,9 @@ static struct rq *finish_task_switch(struct task_struct *prev) vtime_task_switch(prev); perf_event_task_sched_in(prev, current); finish_task(prev); +#ifdef CONFIG_PREEMPT_RT + __preempt_count_add(current->softirq_disable_cnt); +#endif finish_lock_switch(rq); finish_arch_post_lock_switch(); kcov_finish_switch(current);
And I expect a few adjustments in schedule_debug() and atomic checks more generally but that should be it and it would probably be less error-prone. Although I'm probably overlooking other issues on the way.
Thanks.
> > /* > * We do not account for softirq time from ksoftirqd here. > @@ -421,7 +421,7 @@ void vtime_task_switch(struct task_struc > > void vtime_account_irq(struct task_struct *tsk, unsigned int offset) > { > - unsigned int pc = preempt_count() - offset; > + unsigned int pc = irq_count() - offset; > > if (pc & HARDIRQ_OFFSET) { > vtime_account_hardirq(tsk); >
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