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Message-ID: <20121005211234.GA2461@linux.vnet.ibm.com>
Date: Fri, 5 Oct 2012 14:12:34 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Antti P Miettinen <ananaza@....fi>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Add "expedite always" switch
On Fri, Oct 05, 2012 at 09:59:15AM +0300, Antti P Miettinen wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > b. Maybe sysfs setting. Initially, this can be simple "on" and "off",
> > exported with root-only access (like you have above). If more
> > elaborate use cases appear, this might become the adminstrative
> > override. The key point is that this needs to go through some
> > sort of separate function (for example, via the KERNEL_ATTR_RW()
> > macro) so that if it needs to change in the future, we can do
> > the change easily.
>
> I did not really envision the module param to be an application
> interface, more like an administrative interface. For dynamically
> requesting things on/off you probably want something like PM QoS which
> does proper book keeping and consolidation but PM QoS for this feels a
> bit overkill. But I do not think sysfs node is appropriate either for
> an application interface. I guess module_param_cb() would also achieve
> the goal of "doing something" upon parameter change. Anyway - below
> the 0-perm module param + sysfs node version.
Queued, thank you!!! I updated the commit log to read as follows:
Add a module parameter to force use of expedited RCU primitives
There have been some embedded applications that would benefit from
use of expedited grace-period primitives. In some ways, this is
similar to synchronize_net() doing either a normal or an expedited
grace period depending on lock state, but with control outside of
the kernel.
This commit therefore adds rcu_expedited boot and sysfs parameters
that cause the kernel to substitute expedited primitives for the
normal grace-period primitives.
Thanx, Paul
> --Antti
>
> Add a module parameter for always using expedited RCU primitives.
>
> Signed-off-by: Antti P Miettinen <amiettinen@...dia.com>
> ---
> kernel/ksysfs.c | 18 ++++++++++++++++++
> kernel/rcu.h | 2 ++
> kernel/rcupdate.c | 3 +++
> kernel/rcutiny_plugin.h | 5 ++++-
> kernel/rcutree.c | 10 ++++++++--
> kernel/rcutree_plugin.h | 5 ++++-
> kernel/srcu.c | 6 +++++-
> 7 files changed, 44 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
> index 4e316e1..8715a79 100644
> --- a/kernel/ksysfs.c
> +++ b/kernel/ksysfs.c
> @@ -141,6 +141,23 @@ static ssize_t fscaps_show(struct kobject *kobj,
> }
> KERNEL_ATTR_RO(fscaps);
>
> +int rcu_expedited;
> +static ssize_t rcu_expedited_show(struct kobject *kobj,
> + struct kobj_attribute *attr, char *buf)
> +{
> + return sprintf(buf, "%d\n", rcu_expedited);
> +}
> +static ssize_t rcu_expedited_store(struct kobject *kobj,
> + struct kobj_attribute *attr,
> + const char *buf, size_t count)
> +{
> + if (kstrtoint(buf, 0, &rcu_expedited))
> + return -EINVAL;
> +
> + return count;
> +}
> +KERNEL_ATTR_RW(rcu_expedited);
> +
> /*
> * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
> */
> @@ -182,6 +199,7 @@ static struct attribute * kernel_attrs[] = {
> &kexec_crash_size_attr.attr,
> &vmcoreinfo_attr.attr,
> #endif
> + &rcu_expedited_attr.attr,
> NULL
> };
>
> diff --git a/kernel/rcu.h b/kernel/rcu.h
> index 8ba99cd..20dfba5 100644
> --- a/kernel/rcu.h
> +++ b/kernel/rcu.h
> @@ -109,4 +109,6 @@ static inline bool __rcu_reclaim(char *rn, struct rcu_head *head)
> }
> }
>
> +extern int rcu_expedited;
> +
> #endif /* __LINUX_RCU_H */
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index 29ca1c6..a2cf761 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -46,12 +46,15 @@
> #include <linux/export.h>
> #include <linux/hardirq.h>
> #include <linux/delay.h>
> +#include <linux/module.h>
>
> #define CREATE_TRACE_POINTS
> #include <trace/events/rcu.h>
>
> #include "rcu.h"
>
> +module_param(rcu_expedited, int, 0);
> +
> #ifdef CONFIG_PREEMPT_RCU
>
> /*
> diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
> index 3d01902..f85016a 100644
> --- a/kernel/rcutiny_plugin.h
> +++ b/kernel/rcutiny_plugin.h
> @@ -706,7 +706,10 @@ void synchronize_rcu(void)
> return;
>
> /* Once we get past the fastpath checks, same code as rcu_barrier(). */
> - rcu_barrier();
> + if (rcu_expedited)
> + synchronize_rcu_expedited();
> + else
> + rcu_barrier();
> }
> EXPORT_SYMBOL_GPL(synchronize_rcu);
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 4fb2376..744c117 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2221,7 +2221,10 @@ void synchronize_sched(void)
> "Illegal synchronize_sched() in RCU-sched read-side critical section");
> if (rcu_blocking_is_gp())
> return;
> - wait_rcu_gp(call_rcu_sched);
> + if (rcu_expedited)
> + synchronize_sched_expedited();
> + else
> + wait_rcu_gp(call_rcu_sched);
> }
> EXPORT_SYMBOL_GPL(synchronize_sched);
>
> @@ -2242,7 +2245,10 @@ void synchronize_rcu_bh(void)
> "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
> if (rcu_blocking_is_gp())
> return;
> - wait_rcu_gp(call_rcu_bh);
> + if (rcu_expedited)
> + synchronize_rcu_bh_expedited();
> + else
> + wait_rcu_gp(call_rcu_bh);
> }
> EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
>
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index f921154..0e9ca8b 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -679,7 +679,10 @@ void synchronize_rcu(void)
> "Illegal synchronize_rcu() in RCU read-side critical section");
> if (!rcu_scheduler_active)
> return;
> - wait_rcu_gp(call_rcu);
> + if (rcu_expedited)
> + synchronize_rcu_expedited();
> + else
> + wait_rcu_gp(call_rcu);
> }
> EXPORT_SYMBOL_GPL(synchronize_rcu);
>
> diff --git a/kernel/srcu.c b/kernel/srcu.c
> index 97c465e..d56873c 100644
> --- a/kernel/srcu.c
> +++ b/kernel/srcu.c
> @@ -34,6 +34,8 @@
> #include <linux/delay.h>
> #include <linux/srcu.h>
>
> +#include "rcu.h"
> +
> /*
> * Initialize an rcu_batch structure to empty.
> */
> @@ -464,7 +466,9 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
> */
> void synchronize_srcu(struct srcu_struct *sp)
> {
> - __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
> + __synchronize_srcu(sp, rcu_expedited
> + ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
> + : SYNCHRONIZE_SRCU_TRYCOUNT);
> }
> EXPORT_SYMBOL_GPL(synchronize_srcu);
>
> --
> 1.7.4.1
>
--
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