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| author | Frederic Weisbecker <frederic@kernel.org> | 2021-07-06 01:43:43 +0200 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@kernel.org> | 2021-08-06 13:41:49 -0700 |
| commit | 508958259bb3d9ca4ec37f0abdb211e9a6f3daa2 (patch) | |
| tree | df4200f1051475a2abf001e92143d2d288359cd3 /kernel/sched | |
| parent | 8211e922de2854130e3633f52cd4fc2d7817ceb0 (diff) | |
| download | linux-508958259bb3d9ca4ec37f0abdb211e9a6f3daa2.tar.bz2 | |
rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU
The cond_resched() function reports an RCU quiescent state only in
non-preemptible TREE RCU implementation. This commit therefore adds a
comment explaining why cond_resched() does nothing in preemptible kernels.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/sched')
| -rw-r--r-- | kernel/sched/core.c | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 2d9ff40f4661..6a03c3fac55c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7781,6 +7781,17 @@ int __sched __cond_resched(void) preempt_schedule_common(); return 1; } + /* + * In preemptible kernels, ->rcu_read_lock_nesting tells the tick + * whether the current CPU is in an RCU read-side critical section, + * so the tick can report quiescent states even for CPUs looping + * in kernel context. In contrast, in non-preemptible kernels, + * RCU readers leave no in-memory hints, which means that CPU-bound + * processes executing in kernel context might never report an + * RCU quiescent state. Therefore, the following code causes + * cond_resched() to report a quiescent state, but only when RCU + * is in urgent need of one. + */ #ifndef CONFIG_PREEMPT_RCU rcu_all_qs(); #endif |