rcu: Avoid having just-onlined CPU resched itself when RCU is idle
RFC PATCH 1/1 rcu: Avoid having just-onlined CPU resched itself when RCU is idle
CPUs set rdp->qs_pending when coming online to resolve races with grace-period start. However, this means that if RCU is idle, the just-onlined CPU might needlessly send itself resched IPIs. Adjust the online-CPU initialization to avoid this, and also to correctly cause the CPU to respond to the current grace period if needed.
This patch is believed to fix or otherwise suppress problems in
https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the relationship is not apparent to this patch's author. That said, it is hard to argue with test results.
This will be available from -rcu as soon as kernel.org's mirrors update:
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent
rcutree.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)
kernel/rcutree.c +8 -3
@@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)/* Set up local state, ensuring consistent view of global state. */ raw_spin_lock_irqsave (&rnp->lock, flags); - rdp->passed_quiesc =0 ;/* We could be racing with new GP, */ - rdp->qs_pending =1 ;/* so set up to respond to current GP. */ rdp->beenonline =1 ;/* We have now been online. */ rdp->preemptible = preemptible; rdp->qlen_last_fqs_check =0 ;
@@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) rnp->qsmaskinit |= mask; mask = rnp->grpmask;if (rnp == rdp->mynode) { - rdp->gpnum = rnp->completed;/* if GP in progress... */ +/* +* If there is a grace period in progress, we will +* set up to wait for it next time we run the +* RCU core code. +*/ + rdp->gpnum = rnp->completed; rdp->completed = rnp->completed; + rdp->passed_quiesc =0 ; + rdp->qs_pending =0 ; rdp->passed_quiesc_completed = rnp->completed -1 ; }raw_spin_unlock (&rnp->lock);/* irqs already disabled. */
Quoted 15 lines · Paul E. McKenney
CPUs set
rdp->qs_pendingwhen coming online to resolve races with grace-period start. However, this means that if RCU is idle, the just-onlined CPU might needlessly send itself resched IPIs. Adjust the online-CPU initialization to avoid this, and also to correctly cause the CPU to respond to the current grace period if needed.This patch is believed to fix or otherwise suppress problems in
https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the relationship is not apparent to this patch's author. That said, it is hard to argue with test results.This will be available from -rcu as soon as kernel.org's mirrors update:
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.gitrcu/urgent
Quoted 9 lines
Reported-by Nicolas Mailhot <[email protected]>Reported-by Michal Jaegermann <[email protected]>Reported-by Josh Boyer <[email protected]>Signed-off-by Paul E. McKenney <[email protected]>Tested-by Josh Boyer <[email protected]>rcutree.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)
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This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read Documentation/stable_kernel_rules.txt for how to do this properly.
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