On Fri, Jun 12, 2020 at 2:12 AM Kaneda, Erik <erik.kaneda@xxxxxxxxx> wrote:
>
>
>
> > -----Original Message-----
> > From: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
> > Sent: Wednesday, June 10, 2020 5:22 AM
> > To: Williams, Dan J <dan.j.williams@xxxxxxxxx>
> > Cc: Kaneda, Erik <erik.kaneda@xxxxxxxxx>; Wysocki, Rafael J
> > <rafael.j.wysocki@xxxxxxxxx>; Len Brown <lenb@xxxxxxxxxx>; Borislav
> > Petkov <bp@xxxxxxxxx>; Weiny, Ira <ira.weiny@xxxxxxxxx>; James Morse
> > <james.morse@xxxxxxx>; Myron Stowe <myron.stowe@xxxxxxxxxx>;
> > Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>; linux-
> > kernel@xxxxxxxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx; linux-
> > nvdimm@xxxxxxxxxxxx; Moore, Robert <robert.moore@xxxxxxxxx>
> > Subject: [RFT][PATCH 2/3] ACPICA: Remove unused memory mappings on
> > interpreter exit
> >
> > From: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
> >
> > For transient memory opregions that are created dynamically under
> > the namespace and interpreter mutexes and go away quickly, there
> > still is the problem that removing their memory mappings may take
> > significant time and so doing that while holding the mutexes should
> > be avoided.
> >
> > For example, unmapping a chunk of memory associated with a memory
> > opregion in Linux involves running synchronize_rcu_expedited()
> > which really should not be done with the namespace mutex held.
> >
> > To address that problem, notice that the unused memory mappings left
> > behind by the "dynamic" opregions that went away need not be unmapped
> > right away when the opregion is deactivated. Instead, they may be
> > unmapped when exiting the interpreter, after the namespace and
> > interpreter mutexes have been dropped (there's one more place dealing
> > with opregions in the debug code that can be treated analogously).
> >
> > Accordingly, change acpi_ev_system_memory_region_setup() to put
> > the unused mappings into a global list instead of unmapping them
> > right away and add acpi_ev_system_release_memory_mappings() to
> > be called when leaving the interpreter in order to unmap the
> > unused memory mappings in the global list (which is protected
> > by the namespace mutex).
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> > drivers/acpi/acpica/acevents.h | 2 ++
> > drivers/acpi/acpica/dbtest.c | 3 ++
> > drivers/acpi/acpica/evrgnini.c | 51
> > ++++++++++++++++++++++++++++++++--
> > drivers/acpi/acpica/exutils.c | 3 ++
> > drivers/acpi/acpica/utxface.c | 23 +++++++++++++++
> > include/acpi/acpixf.h | 1 +
> > 6 files changed, 80 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/acpi/acpica/acevents.h b/drivers/acpi/acpica/acevents.h
> > index 79f292687bd6..463eb9124765 100644
> > --- a/drivers/acpi/acpica/acevents.h
> > +++ b/drivers/acpi/acpica/acevents.h
> > @@ -197,6 +197,8 @@ acpi_ev_execute_reg_method(union
> > acpi_operand_object *region_obj, u32 function);
> > /*
> > * evregini - Region initialization and setup
> > */
> > +void acpi_ev_system_release_memory_mappings(void);
> > +
> > acpi_status
> > acpi_ev_system_memory_region_setup(acpi_handle handle,
> > u32 function,
> > diff --git a/drivers/acpi/acpica/dbtest.c b/drivers/acpi/acpica/dbtest.c
> > index 6db44a5ac786..7dac6dae5c48 100644
> > --- a/drivers/acpi/acpica/dbtest.c
> > +++ b/drivers/acpi/acpica/dbtest.c
> > @@ -8,6 +8,7 @@
> > #include <acpi/acpi.h>
> > #include "accommon.h"
> > #include "acdebug.h"
> > +#include "acevents.h"
> > #include "acnamesp.h"
> > #include "acpredef.h"
> > #include "acinterp.h"
> > @@ -768,6 +769,8 @@ acpi_db_test_field_unit_type(union
> > acpi_operand_object *obj_desc)
> > acpi_ut_release_mutex(ACPI_MTX_NAMESPACE);
> > acpi_ut_release_mutex(ACPI_MTX_INTERPRETER);
> >
> > + acpi_ev_system_release_memory_mappings();
> > +
> > bit_length = obj_desc->common_field.bit_length;
> > byte_length =
> > ACPI_ROUND_BITS_UP_TO_BYTES(bit_length);
> >
> > diff --git a/drivers/acpi/acpica/evrgnini.c b/drivers/acpi/acpica/evrgnini.c
> > index 48a5e6eaf9b9..946c4eef054d 100644
> > --- a/drivers/acpi/acpica/evrgnini.c
> > +++ b/drivers/acpi/acpica/evrgnini.c
> > @@ -16,6 +16,52 @@
> > #define _COMPONENT ACPI_EVENTS
> > ACPI_MODULE_NAME("evrgnini")
> >
> > +#ifdef ACPI_OS_MAP_MEMORY_FAST_PATH
> > +static struct acpi_mem_mapping *unused_memory_mappings;
> > +
> > +/*********************************************************
> > **********************
> > + *
> > + * FUNCTION: acpi_ev_system_release_memory_mappings
> > + *
> > + * PARAMETERS: None
> > + *
> > + * RETURN: None
> > + *
> > + * DESCRIPTION: Release all of the unused memory mappings in the queue
> > + * under the interpreter mutex.
> > + *
> > +
> > **********************************************************
> > ********************/
> > +void acpi_ev_system_release_memory_mappings(void)
> > +{
> > + struct acpi_mem_mapping *mapping;
> > +
> > +
> > ACPI_FUNCTION_TRACE(acpi_ev_system_release_memory_mappin
> > gs);
> > +
> > + acpi_ut_acquire_mutex(ACPI_MTX_NAMESPACE);
> > +
> > + while (unused_memory_mappings) {
> > + mapping = unused_memory_mappings;
> > + unused_memory_mappings = mapping->next;
> > +
> > + acpi_ut_release_mutex(ACPI_MTX_NAMESPACE);
> > +
> > + acpi_os_unmap_memory(mapping->logical_address,
> > mapping->length);
>
> acpi_os_unmap_memory calls synchronize_rcu_expedited(). I'm no RCU expert but the
> definition of this function states:
>
> * Although this is a great improvement over previous expedited
> * implementations, it is still unfriendly to real-time workloads, so is
> * thus not recommended for any sort of common-case code. In fact, if
> * you are using synchronize_rcu_expedited() in a loop, please restructure
> * your code to batch your updates, and then use a single synchronize_rcu()
> * instead.
If this really ends up being a loop, the code without this patch will
also call synchronize_rcu_expedited() in a loop, but indirectly and
under the namespace and interpreter mutexes.
While I agree that this is still somewhat suboptimal, improving this
would require more changes in the OSL code.
Cheers!
