From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu <rcu@vger.kernel.org>
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 20:57:10 -0700 [thread overview]
Message-ID: <20190812035710.GF28441@linux.ibm.com> (raw)
In-Reply-To: <20190812014053.GD128944@google.com>
On Sun, Aug 11, 2019 at 09:40:53PM -0400, Joel Fernandes wrote:
> 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?
Looks valid to me! There might well be other scenarios as well, but
only one is required to justify the resched_cpu().
Thanx, Paul
next prev parent reply other threads:[~2019-08-12 3:57 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-11 18:08 need_heavy_qs flag for PREEMPT=y kernels 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
2019-08-12 3:57 ` Paul E. McKenney [this message]
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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