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Date:	Sun, 9 Nov 2014 14:00:38 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	linux-acpi@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Tom Boshoven <tomboshoven@...il.com>, x86@...nel.org,
	Josh Triplett <josh@...htriplett.org>,
	Alexander Monakov <amonakov@...il.com>
Subject: Re: [PATCH v2] ACPI/osl: speedup grace period in acpi_os_map_cleanup

On Sun, Nov 09, 2014 at 01:53:37PM +0400, Konstantin Khlebnikov wrote:
> ACPI maintains cache of ioremap regions to speed up operations and
> access to them from irq context where ioremap() calls aren't allowed.
> This code abuses synchronize_rcu() on unmap path for synchronization
> with fast-path in acpi_os_read/write_memory which uses this cache.
> 
> Since v3.10 CPUs are allowed to enter idle state even if they have RCU
> callbacks queued, see commit c0f4dfd4f90f1667d234d21f15153ea09a2eaa66
> ("rcu: Make RCU_FAST_NO_HZ take advantage of numbered callbacks").
> That change caused problems with nvidia proprietary driver which calls
> acpi_os_map/unmap_generic_address several times during initialization.
> Each unmap calls synchronize_rcu and adds significant delay. Totally
> initialization is slowed for a couple of seconds and that is enough to
> trigger timeout in hardware, gpu decides to "fell off the bus". Widely
> spread workaround is reducing "rcu_idle_gp_delay" from 4 to 1 jiffy.
> 
> This patch replaces synchronize_rcu() with synchronize_rcu_expedited()
> which is much faster.
> 
> Signed-off-by: Konstantin Khlebnikov <koct9i@...il.com>
> Reported-and-tested-by: Alexander Monakov <amonakov@...il.com>
> Cc: Tom Boshoven <tomboshoven@...il.com>
> Link: https://devtalk.nvidia.com/default/topic/567297/linux/linux-3-10-driver-crash/

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  drivers/acpi/osl.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
> index 9964f70..217713c 100644
> --- a/drivers/acpi/osl.c
> +++ b/drivers/acpi/osl.c
> @@ -436,7 +436,7 @@ static void acpi_os_drop_map_ref(struct acpi_ioremap *map)
>  static void acpi_os_map_cleanup(struct acpi_ioremap *map)
>  {
>  	if (!map->refcount) {
> -		synchronize_rcu();
> +		synchronize_rcu_expedited();
>  		acpi_unmap(map->phys, map->virt);
>  		kfree(map);
>  	}
> 
> --
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