From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Paul McKenney <paulmck@kernel.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [patch V2 2/9] irqtime: Make accounting correct on RT
Date: Mon, 7 Dec 2020 01:23:43 +0100 [thread overview]
Message-ID: <20201207002343.GA113660@lothringen> (raw)
In-Reply-To: <20201204170804.889561591@linutronix.de>
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);
>
next prev parent reply other threads:[~2020-12-07 0:24 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-04 17:01 [patch V2 0/9] softirq: Make it RT aware Thomas Gleixner
2020-12-04 17:01 ` [patch V2 1/9] softirq: Add RT specific softirq accounting Thomas Gleixner
2020-12-07 13:06 ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 2/9] irqtime: Make accounting correct on RT Thomas Gleixner
2020-12-07 0:23 ` Frederic Weisbecker [this message]
2020-12-07 0:57 ` Thomas Gleixner
2020-12-07 1:14 ` Frederic Weisbecker
2020-12-07 13:27 ` Frederic Weisbecker
2020-12-07 14:44 ` Thomas Gleixner
2020-12-04 17:01 ` [patch V2 3/9] softirq: Move various protections into inline helpers Thomas Gleixner
2020-12-07 13:37 ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 4/9] softirq: Make softirq control and processing RT aware Thomas Gleixner
2020-12-07 14:16 ` Frederic Weisbecker
2020-12-07 15:08 ` Thomas Gleixner
2020-12-08 0:08 ` Frederic Weisbecker
2020-12-09 10:11 ` Peter Zijlstra
2020-12-09 12:36 ` Thomas Gleixner
2020-12-09 12:42 ` Peter Zijlstra
2020-12-09 13:30 ` Thomas Gleixner
2020-12-09 10:34 ` Peter Zijlstra
2020-12-04 17:01 ` [patch V2 5/9] tick/sched: Prevent false positive softirq pending warnings on RT Thomas Gleixner
2020-12-08 12:23 ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 6/9] rcu: Prevent false positive softirq warning " Thomas Gleixner
2020-12-04 17:59 ` Paul E. McKenney
2020-12-04 17:01 ` [patch V2 7/9] softirq: Replace barrier() with cpu_relax() in tasklet_unlock_wait() Thomas Gleixner
2020-12-07 11:39 ` Peter Zijlstra
2020-12-07 15:21 ` Thomas Gleixner
2020-12-04 17:01 ` [patch V2 8/9] tasklets: Use static inlines for stub implementations Thomas Gleixner
2020-12-04 17:02 ` [patch V2 9/9] tasklets: Prevent kill/unlock_wait deadlock on RT Thomas Gleixner
2020-12-07 11:47 ` Peter Zijlstra
2020-12-07 14:00 ` Sebastian Andrzej Siewior
2020-12-07 14:27 ` Peter Zijlstra
2020-12-07 17:55 ` Thomas Gleixner
2020-12-07 15:22 ` Thomas Gleixner
2020-12-07 15:39 ` Sebastian Andrzej Siewior
2020-12-07 17:49 ` Thomas Gleixner
2020-12-07 17:50 ` Thomas Gleixner
2020-12-06 10:05 ` [patch V2 0/9] softirq: Make it RT aware Sebastian Andrzej Siewior
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