linux-kernel - Re: [PATCH v2] ACPI/osl: speedup grace period in acpi_os_map

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Date:	Fri, 14 Nov 2014 23:48:30 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Konstantin Khlebnikov <koct9i@...il.com>,
	linux-acpi@...r.kernel.org, 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 Sunday, November 09, 2014 02:00:38 PM Paul E. McKenney wrote:
> 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>

Patch queued up for 3.19, thanks!

> 
> > ---
> >  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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> 
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-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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