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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-07-09 22:47:30 +0200 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-08-31 01:03:39 +0200 |
commit | 92aa39e9dc77481b90cbef25e547d66cab901496 (patch) | |
tree | 71b231d7fcddf06c0d6f8a4b6e8ab414ac0f0756 /kernel | |
parent | rcu: Inline _rcu_barrier() into its sole remaining caller (diff) | |
download | linux-92aa39e9dc77481b90cbef25e547d66cab901496.tar.xz linux-92aa39e9dc77481b90cbef25e547d66cab901496.zip |
rcu: Make need_resched() respond to urgent RCU-QS needs
The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
need for an RCU quiescent state from the force-quiescent-state processing
within the grace-period kthread to context switches and to cond_resched().
Unfortunately, such urgent needs are not communicated to need_resched(),
which is sometimes used to decide when to invoke cond_resched(), for
but one example, within the KVM vcpu_run() function. As of v4.15, this
can result in synchronize_sched() being delayed by up to ten seconds,
which can be problematic, to say nothing of annoying.
This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
rcu_check_callbacks(), which is invoked from the scheduling-clock
interrupt handler. If the current task is not an idle task and is
not executing in usermode, a context switch is forced, and either way,
the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current
task is an idle task, then RCU's dyntick-idle code will detect the
quiescent state, so no further action is required. Similarly, if the
task is executing in usermode, other code in rcu_check_callbacks() and
its called functions will report the corresponding quiescent state.
Reported-by: Marius Hillenbrand <mhillenb@amazon.de>
Reported-by: David Woodhouse <dwmw2@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/rcu/tree.c | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index ce16b8da2c6f..f47ac7a4719f 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2471,6 +2471,15 @@ void rcu_check_callbacks(int user) { trace_rcu_utilization(TPS("Start scheduler-tick")); raw_cpu_inc(rcu_data.ticks_this_gp); + /* The load-acquire pairs with the store-release setting to true. */ + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) { + /* Idle and userspace execution already are quiescent states. */ + if (!is_idle_task(current) && !user) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); + } rcu_flavor_check_callbacks(user); if (rcu_pending()) invoke_rcu_core(); |