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From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu <rcu@vger.kernel.org>
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 21:40:53 -0400
Message-ID: <20190812014053.GD128944@google.com> (raw)
In-Reply-To: <20190812012431.GC128944@google.com>

On Sun, Aug 11, 2019 at 09:24:31PM -0400, Joel Fernandes wrote:
> On Sun, Aug 11, 2019 at 04:30:24PM -0700, Paul E. McKenney wrote:
[snip]
> > Next question:  Why does rcu_implicit_dynticks_qs() check only for
> > tick_nohz_full_cpu() and not also IS_ENABLED(CONFIG_PREEMPT)?  After
> > all, a nohz_full CPU in a !CONFIG_PREEMPT kernel should be able to
> > rely on cond_resched(), right?
> > 
> > Should this change?  Why or why not?
> 
> Let me think more about this :) I have an answer in mind but I will think a
> bit more about it and responsd :)

It should not change. That's because (as somewhat mentioned in the comments),
some code paths in the kernel check need_resched() before invoking
cond_resched(). So even with PREEMPT=n having the help of cond_resched(), the
cond_resched() may actually not even be invoked.  So in this case, the
resched_cpu() from rcu_implicit_dynticks_qs() does the needful by setting the
rescheduling flags on the CPU, so that cond_resched() on those CPUs actually
get called. Is that a correct analysis?

thanks,

 - Joel


  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-11 18:08 Joel Fernandes
2019-08-11 18:34 ` Joel Fernandes
2019-08-11 21:16   ` Paul E. McKenney
2019-08-11 21:25     ` Joel Fernandes
2019-08-11 23:30       ` Paul E. McKenney
2019-08-12  1:24         ` Joel Fernandes
2019-08-12  1:40           ` Joel Fernandes [this message]
2019-08-12  3:57             ` Paul E. McKenney
2019-08-11 21:13 ` Paul E. McKenney
2019-08-12  3:21   ` Joel Fernandes
2019-08-12  3:53     ` Paul E. McKenney
2019-08-12 21:20       ` Joel Fernandes
2019-08-12 23:01         ` Paul E. McKenney
2019-08-13  1:02           ` Joel Fernandes
2019-08-13  1:05             ` Joel Fernandes
2019-08-13  2:28               ` Paul E. McKenney
2019-08-13  2:27             ` Paul E. McKenney
2019-08-13  2:50               ` Paul E. McKenney
2019-08-15 17:17             ` Paul E. McKenney
2019-08-15 20:04               ` Joel Fernandes
2019-08-15 20:31                 ` Paul E. McKenney
2019-08-15 21:22                   ` Joel Fernandes
2019-08-15 21:27                     ` Joel Fernandes
2019-08-15 21:34                       ` Joel Fernandes
2019-08-15 21:57                         ` Paul E. McKenney
2019-08-15 21:45                     ` Paul E. McKenney
2019-08-16  0:02                       ` Joel Fernandes
2019-08-19 12:34                         ` Frederic Weisbecker
2019-08-19 12:09                   ` Frederic Weisbecker
2019-08-19 16:57                   ` Frederic Weisbecker
2019-08-19 22:31                     ` Paul E. McKenney

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