Re: [PATCH] rcu: Add "expedite always" switch

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Oct 05, 2012 at 09:59:15AM +0300, Antti P Miettinen wrote:
> From: "Paul E. McKenney" <paulmck@linux.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@nvidia.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
>