Re: [PATCH 1/3] sched/core: Remove the unnecessary need_resched() check in nohz_csd_func()

[K Prateek Nayak <kprateek.nayak@amd.com>]

(+ Thomas, Sebastian, Christoph)

Hello everyone,

Adding folks who were cc'd on
https://lore.kernel.org/all/20220413133024.356509586@linutronix.de/

On 7/10/2024 2:32 PM, K Prateek Nayak wrote:
> The need_resched() check currently in nohz_csd_func() can be tracked
> to have been added in scheduler_ipi() back in 2011 via commit
> ca38062e57e9 ("sched: Use resched IPI to kick off the nohz idle balance")
> 
> Since then, it has travelled quite a bit but it seems like an idle_cpu()
> check currently is sufficient to detect the need to bail out from an
> idle load balancing. To justify this removal, consider all the following
> case where an idle load balancing could race with a task wakeup:
> 
> o Since commit f3dd3f674555b ("sched: Remove the limitation of WF_ON_CPU
>    on wakelist if wakee cpu is idle") a target perceived to be idle
>    (target_rq->nr_running == 0) will return true for
>    ttwu_queue_cond(target) which will offload the task wakeup to the idle
>    target via an IPI.
> 
>    In all such cases target_rq->ttwu_pending will be set to 1 before
>    queuing the wake function.
> 
>    If an idle load balance races here, following scenarios are possible:
> 
>    - The CPU is not in TIF_POLLING_NRFLAG mode in which case an actual
>      IPI is sent to the CPU to wake it out of idle. If the
>      nohz_csd_func() queues before sched_ttwu_pending(), the idle load
>      balance will bail out since idle_cpu(target) returns 0 since
>      target_rq->ttwu_pending is 1. If the nohz_csd_func() is queued after
>      sched_ttwu_pending() it should see rq->nr_running to be non-zero and
>      bail out of idle load balancing.
> 
>    - The CPU is in TIF_POLLING_NRFLAG mode and instead of an actual IPI,
>      the sender will simply set TIF_NEED_RESCHED for the target to put it
>      out of idle and flush_smp_call_function_queue() in do_idle() will
>      execute the call function. Depending on the ordering of the queuing
>      of nohz_csd_func() and sched_ttwu_pending(), the idle_cpu() check in
>      nohz_csd_func() should either see target_rq->ttwu_pending = 1 or
>      target_rq->nr_running to be non-zero if there is a genuine task
>      wakeup racing with the idle load balance kick.
> 
> o The waker CPU perceives the target CPU to be busy
>    (targer_rq->nr_running != 0) but the CPU is in fact going idle and due
>    to a series of unfortunate events, the system reaches a case where the
>    waker CPU decides to perform the wakeup by itself in ttwu_queue() on
>    the target CPU but target is concurrently selected for idle load
>    balance (Can this happen? I'm not sure, but we'll consider its
>    possibility to estimate the worst case scenario).
> 
>    ttwu_do_activate() calls enqueue_task() which would increment
>    "rq->nr_running" post which it calls wakeup_preempt() which is
>    responsible for setting TIF_NEED_RESCHED (via a resched IPI or by
>    setting TIF_NEED_RESCHED on a TIF_POLLING_NRFLAG idle CPU) The key
>    thing to note in this case is that rq->nr_running is already non-zero
>    in case of a wakeup before TIF_NEED_RESCHED is set which would
>    lead to idle_cpu() check returning false.
> 
> In all cases, it seems that need_resched() check is unnecessary when
> checking for idle_cpu() first since an impending wakeup racing with idle
> load balancer will either set the "rq->ttwu_pending" or indicate a newly
> woken task via "rq->nr_running".
> 
> Chasing the reason why this check might have existed in the first place,
> I came across  Peter's suggestion on the fist iteration of Suresh's
> patch from 2011 [1] where the condition to raise the SCHED_SOFTIRQ was:
> 
> 	sched_ttwu_do_pending(list);
> 
> 	if (unlikely((rq->idle == current) &&
> 	    rq->nohz_balance_kick &&
> 	    !need_resched()))
> 		raise_softirq_irqoff(SCHED_SOFTIRQ);
> 
> However, since this was preceded by sched_ttwu_do_pending() which is
> equivalent of sched_ttwu_pending() in the current upstream kernel, the
> need_resched() check was necessary to catch a newly queued task. Peter
> suggested modifying it to:
> 
> 	if (idle_cpu() && rq->nohz_balance_kick && !need_resched())
> 		raise_softirq_irqoff(SCHED_SOFTIRQ);
> 
> where idle_cpu() seems to have replaced "rq->idle == current" check.
> However, even back then, the idle_cpu() check would have been sufficient
> to have caught the enqueue of a new task and since commit b2a02fc43a1f
> ("smp: Optimize send_call_function_single_ipi()") overloads the
> interpretation of TIF_NEED_RESCHED for TIF_POLLING_NRFLAG idling, remove
> the need_resched() check in nohz_csd_func() to raise SCHED_SOFTIRQ based
> on Peter's suggestion.
> 
> Link: https://lore.kernel.org/all/1317670590.20367.38.camel@twins/ [1]
> Link: https://lore.kernel.org/lkml/20240615014521.GR8774@noisy.programming.kicks-ass.net/
> Fixes: b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()")

Turns out the above commit + commit 1a90bfd22020 ("smp: Make softirq
handling RT safe in flush_smp_call_function_queue()") will trigger the
WARN_ON_ONCE() in do_softirq_post_smp_call_flush for RT kernels after
this change since the nohz_csd_func() will now raise a SCHED_SOFTIRQ
to trigger the idle balance and is executed from
flush_smp_call_function_queue() in do_idle().

I noticed the following splat early into the boot during my testing
of the series:

     ------------[ cut here ]------------
     WARNING: CPU: 4 PID: 0 at kernel/softirq.c:326 do_softirq_post_smp_call_flush+0x1a/0x40
     Modules linked in:
     CPU: 4 PID: 0 Comm: swapper/4 Not tainted 6.10.0-rc6-rt11-test-rt+ #1160
     Hardware name: Dell Inc. PowerEdge R6525/024PW1, BIOS 2.7.3 03/30/2022
     RIP: 0010:do_softirq_post_smp_call_flush+0x1a/0x40
     Code: ...
     RSP: 0018:ffffb3ae003a7eb8 EFLAGS: 00010002
     RAX: 0000000000000080 RBX: 0000000000000282 RCX: 0000000000000007
     RDX: 0000000000000000 RSI: ffff9fc3fb4492e0 RDI: 0000000000000000
     RBP: 0000000000000000 R08: 0000000000000000 R09: 00000000ffffffff
     R10: 000000000000009b R11: ffff9f8586e2d4d0 R12: 0000000000000000
     R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
     FS:  0000000000000000(0000) GS:ffff9fc3fb400000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 0000000000000000 CR3: 000000807d470001 CR4: 0000000000770ef0
     PKRU: 55555554
     Call Trace:
      <TASK>
      ? __warn+0x88/0x180
      ? do_softirq_post_smp_call_flush+0x1a/0x40
      ? report_bug+0x18e/0x1a0
      ? handle_bug+0x42/0x70
      ? exc_invalid_op+0x18/0x70
      ? asm_exc_invalid_op+0x1a/0x20
      ? do_softirq_post_smp_call_flush+0x1a/0x40
      ? srso_alias_return_thunk+0x5/0xfbef5
      flush_smp_call_function_queue+0x7a/0x90
      do_idle+0x15f/0x270
      cpu_startup_entry+0x29/0x30
      start_secondary+0x12b/0x160
      common_startup_64+0x13e/0x141
      </TASK>
     ---[ end trace 0000000000000000 ]---

which points to:

     WARN_ON_ONCE(was_pending != local_softirq_pending())

Since MWAIT based idling on x86 sets the TIF_POLLING_NRFLAG, IPIs to
an idle CPU are optimized out by commit b2a02fc43a1f ("smp: Optimize
send_call_function_single_ipi()") and the logic instead relies on
flush_smp_call_function_queue() in the idle exit path to execute the
SMP-call-function. This previously went undetected since the sender of
IPI sets the TIF_NEED_RESCHED bit which would have tripped the
need_resched() check in nohz_csd_func() and prevented it from raising
the SOFTIRQ.

Would it be okay to allow raising a SCHED_SOFTIRQ from
flush_smp_call_function_queue() on PREEMPT_RT kernels? Something like:

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 8308687fc7b9..d8ce76e6e318 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -314,17 +314,24 @@ static inline void invoke_softirq(void)
  		wakeup_softirqd();
  }
  
+#define SCHED_SOFTIRQ_MASK	BIT(SCHED_SOFTIRQ)
+
  /*
   * flush_smp_call_function_queue() can raise a soft interrupt in a function
- * call. On RT kernels this is undesired and the only known functionality
- * in the block layer which does this is disabled on RT. If soft interrupts
- * get raised which haven't been raised before the flush, warn so it can be
+ * call. On RT kernels this is undesired and the only known functionalities
+ * are in the block layer which is disabled on RT, and in the scheduler for
+ * idle load balancing. If soft interrupts get raised which haven't been
+ * raised before the flush, warn if it is not a SCHED_SOFTIRQ so it can be
   * investigated.
   */
  void do_softirq_post_smp_call_flush(unsigned int was_pending)
  {
-	if (WARN_ON_ONCE(was_pending != local_softirq_pending()))
+	unsigned int is_pending = local_softirq_pending();
+
+	if (unlikely(was_pending != is_pending)) {
+		WARN_ON_ONCE(was_pending != (is_pending & ~SCHED_SOFTIRQ_MASK));
  		invoke_softirq();
+	}
  }
  
  #else /* CONFIG_PREEMPT_RT */
--

With the above diff, I do not see the splat I was seeing initially. If
there are no strong objections, I can fold in the above diff in v2.
-- 
Thanks and Regards,
Prateek

> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>   kernel/sched/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0935f9d4bb7b..1e0c77eac65a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1205,7 +1205,7 @@ static void nohz_csd_func(void *info)
>   	WARN_ON(!(flags & NOHZ_KICK_MASK));
>   
>   	rq->idle_balance = idle_cpu(cpu);
> -	if (rq->idle_balance && !need_resched()) {
> +	if (rq->idle_balance) {
>   		rq->nohz_idle_balance = flags;
>   		raise_softirq_irqoff(SCHED_SOFTIRQ);
>   	}