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From: Joel Fernandes <joel@joelfernandes.org>
To: rcu@vger.kernel.org, paulmck@linux.ibm.com
Subject: need_heavy_qs flag for PREEMPT=y kernels
Date: Sun, 11 Aug 2019 14:08:52 -0400	[thread overview]
Message-ID: <20190811180852.GA128944@google.com> (raw)

Hi Paul, everyone,

I noticed on reading code that the need_heavy_qs check and
rcu_momentary_dyntick_idle() is only called for !PREEMPT kernels. Don't we
need to call this for PREEMPT kernels for the benefit of nohz_full CPUs?

Consider the following events:
1. Kernel is PREEMPT=y configuration.
2. CPU 2 is a nohz_full CPU running only a single task and the tick is off.
3. CPU 2 is running only in kernel mode and does not enter user mode or idle.
4. Grace period thread running on CPU 3 enter the fqs loop.
5. Enough time passes and it sets the need_heavy_qs for CPU2.
6. CPU 2 is still in kernel mode but does cond_resched().
7. cond_resched() does not call rcu_momentary_dyntick_idle() because PREEMPT=y.

Is 7. not calling rcu_momentary_dyntick_idle() a lost opportunity for the FQS
loop to detect that the CPU has crossed a quiescent point?

Is this done so that cond_resched() is fast for PREEMPT=y kernels?

thanks,

 - Joel



             reply	other threads:[~2019-08-11 18:08 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-11 18:08 Joel Fernandes [this message]
2019-08-11 18:34 ` need_heavy_qs flag for PREEMPT=y kernels 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
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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