* rcu: Avoid irq disable in rcu_cpu_kthread
@ 2013-12-05 21:06 Christoph Lameter
2013-12-10 0:05 ` Paul E. McKenney
0 siblings, 1 reply; 3+ messages in thread
From: Christoph Lameter @ 2013-12-05 21:06 UTC (permalink / raw)
To: Paul E. McKenney; +Cc: linux-kernel
Once we have the per cpu patchset merged we could do the following [it
even works without that patchset but the __this_cpu ops will not do
preemption checks]. Would this work?
Subject: rcu: Avoid irq disable in rcu_cpu_kthread
The use of this_cpu ops avoids numerous address calculations
and allows to avoid the irq enable/disable sequence through a
low latency non locking this_cpu_xchg.
Signed-off-by: Christoph Lameter <cl@linux.com>
Index: linux/kernel/rcu/tree_plugin.h
===================================================================
--- linux.orig/kernel/rcu/tree_plugin.h 2013-12-03 11:32:23.322999660 -0600
+++ linux/kernel/rcu/tree_plugin.h 2013-12-03 11:32:23.312999941 -0600
@@ -1417,33 +1417,29 @@ static int rcu_cpu_kthread_should_run(un
*/
static void rcu_cpu_kthread(unsigned int cpu)
{
- unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
- char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
+ char work;
int spincnt;
for (spincnt = 0; spincnt < 10; spincnt++) {
trace_rcu_utilization(TPS("Start CPU kthread@rcu_wait"));
local_bh_disable();
- *statusp = RCU_KTHREAD_RUNNING;
- this_cpu_inc(rcu_cpu_kthread_loops);
- local_irq_disable();
- work = *workp;
- *workp = 0;
- local_irq_enable();
+ __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_RUNNING);
+ __this_cpu_inc(rcu_cpu_kthread_loops);
+ work = this_cpu_xchg(rcu_cpu_has_work, 0);
if (work)
rcu_kthread_do_work();
local_bh_enable();
- if (*workp == 0) {
+ if (__this_cpu_read(rcu_cpu_has_work) == 0) {
trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
- *statusp = RCU_KTHREAD_WAITING;
+ __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
return;
}
}
- *statusp = RCU_KTHREAD_YIELDING;
+ __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_YIELDING);
trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
schedule_timeout_interruptible(2);
trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
- *statusp = RCU_KTHREAD_WAITING;
+ __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
}
/*
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: rcu: Avoid irq disable in rcu_cpu_kthread
2013-12-05 21:06 rcu: Avoid irq disable in rcu_cpu_kthread Christoph Lameter
@ 2013-12-10 0:05 ` Paul E. McKenney
2013-12-10 15:24 ` Christoph Lameter
0 siblings, 1 reply; 3+ messages in thread
From: Paul E. McKenney @ 2013-12-10 0:05 UTC (permalink / raw)
To: Christoph Lameter; +Cc: linux-kernel
On Thu, Dec 05, 2013 at 09:06:55PM +0000, Christoph Lameter wrote:
> Once we have the per cpu patchset merged we could do the following [it
> even works without that patchset but the __this_cpu ops will not do
> preemption checks]. Would this work?
Looks plausible at first glance. But are you really seeing performance
issues with this code? It is only compiled into the kernel when you build
with CONFIG_RCU_BOOST=y -- are you actually using that for your workloads?
Thanx, Paul
> Subject: rcu: Avoid irq disable in rcu_cpu_kthread
>
> The use of this_cpu ops avoids numerous address calculations
> and allows to avoid the irq enable/disable sequence through a
> low latency non locking this_cpu_xchg.
>
> Signed-off-by: Christoph Lameter <cl@linux.com>
>
> Index: linux/kernel/rcu/tree_plugin.h
> ===================================================================
> --- linux.orig/kernel/rcu/tree_plugin.h 2013-12-03 11:32:23.322999660 -0600
> +++ linux/kernel/rcu/tree_plugin.h 2013-12-03 11:32:23.312999941 -0600
> @@ -1417,33 +1417,29 @@ static int rcu_cpu_kthread_should_run(un
> */
> static void rcu_cpu_kthread(unsigned int cpu)
> {
> - unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
> - char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
> + char work;
> int spincnt;
>
> for (spincnt = 0; spincnt < 10; spincnt++) {
> trace_rcu_utilization(TPS("Start CPU kthread@rcu_wait"));
> local_bh_disable();
> - *statusp = RCU_KTHREAD_RUNNING;
> - this_cpu_inc(rcu_cpu_kthread_loops);
> - local_irq_disable();
> - work = *workp;
> - *workp = 0;
> - local_irq_enable();
> + __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_RUNNING);
> + __this_cpu_inc(rcu_cpu_kthread_loops);
> + work = this_cpu_xchg(rcu_cpu_has_work, 0);
> if (work)
> rcu_kthread_do_work();
> local_bh_enable();
> - if (*workp == 0) {
> + if (__this_cpu_read(rcu_cpu_has_work) == 0) {
> trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
> - *statusp = RCU_KTHREAD_WAITING;
> + __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
> return;
> }
> }
> - *statusp = RCU_KTHREAD_YIELDING;
> + __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_YIELDING);
> trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
> schedule_timeout_interruptible(2);
> trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
> - *statusp = RCU_KTHREAD_WAITING;
> + __this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
> }
>
> /*
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: rcu: Avoid irq disable in rcu_cpu_kthread
2013-12-10 0:05 ` Paul E. McKenney
@ 2013-12-10 15:24 ` Christoph Lameter
0 siblings, 0 replies; 3+ messages in thread
From: Christoph Lameter @ 2013-12-10 15:24 UTC (permalink / raw)
To: Paul E. McKenney; +Cc: linux-kernel
On Mon, 9 Dec 2013, Paul E. McKenney wrote:
> On Thu, Dec 05, 2013 at 09:06:55PM +0000, Christoph Lameter wrote:
> > Once we have the per cpu patchset merged we could do the following [it
> > even works without that patchset but the __this_cpu ops will not do
> > preemption checks]. Would this work?
>
> Looks plausible at first glance. But are you really seeing performance
> issues with this code? It is only compiled into the kernel when you build
> with CONFIG_RCU_BOOST=y -- are you actually using that for your workloads?
I have not done any benchmarking. Just looking for more use cases for the
this_cpu ops. There is a lot of use of per cpu operations in the rcu code
which seems to be areas in which these operations can help.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2013-12-05 21:06 rcu: Avoid irq disable in rcu_cpu_kthread Christoph Lameter
2013-12-10 0:05 ` Paul E. McKenney
2013-12-10 15:24 ` Christoph Lameter
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