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From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu@vger.kernel.org
Subject: Re: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 23:21:42 -0400
Message-ID: <20190812032142.GA171001@google.com> (raw)
In-Reply-To: <20190811211318.GX28441@linux.ibm.com>

On Sun, Aug 11, 2019 at 02:13:18PM -0700, Paul E. McKenney wrote:
> This leaves NO_HZ_FULL=y&&PREEMPT=y kernels.  In that case, RCU is
> more aggressive about using resched_cpu() on CPUs that have not yet
> reported a quiescent state for the current grace period.

Just wanted to ask something - how does resched_cpu() help for this case?

Consider a nohz_full CPU and a PREEMPT=y kernel. Say a single task is running
in kernel mode with scheduler tick off. As we discussed, we have no help from
cond_resched() (since its a PREEMPT=y kernel).  Because enough time has
passed (jtsq*3), we send the CPU a re-scheduling IPI.

This seems not that useful. Even if we enter the scheduler due to the
rescheduling flags set on that CPU, nothing will do the rcu_report_qs_rdp()
or rcu_report_qs_rnp() on those CPUs, which are needed to propagate the
quiescent state to the leaf node. Neither will anything to do a
rcu_momentary_dyntick_idle() for that CPU. Without this, the grace period
will still end up getting blocked.

Could you clarify which code paths on a nohz_full CPU running PREEMPT=y
kernel actually helps to end the grace period when we call resched_cpu() on
it?  Don't we need atleast do a rcu_momentary_dyntick_idle() from the
scheduler IPI handler or from resched_cpu() for the benefit of a nohz_full
CPU? Maybe I should do an experiment to see this all play out.

And I need to write down everything I learnt today before I go crazy... ;-)

  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
2019-08-12  3:57             ` Paul E. McKenney
2019-08-11 21:13 ` Paul E. McKenney
2019-08-12  3:21   ` Joel Fernandes [this message]
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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