linux-kernel - Re: [RFT][PATCH 2/3] ACPICA: Remove unused memory mappings on interpreter exit

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Date:   Fri, 12 Jun 2020 14:05:01 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Kaneda, Erik" <erik.kaneda@...el.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Len Brown <lenb@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        James Morse <james.morse@....com>,
        Myron Stowe <myron.stowe@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Moore, Robert" <robert.moore@...el.com>
Subject: Re: [RFT][PATCH 2/3] ACPICA: Remove unused memory mappings on
 interpreter exit

On Fri, Jun 12, 2020 at 2:12 AM Kaneda, Erik <erik.kaneda@...el.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Rafael J. Wysocki <rjw@...ysocki.net>
> > Sent: Wednesday, June 10, 2020 5:22 AM
> > To: Williams, Dan J <dan.j.williams@...el.com>
> > Cc: Kaneda, Erik <erik.kaneda@...el.com>; Wysocki, Rafael J
> > <rafael.j.wysocki@...el.com>; Len Brown <lenb@...nel.org>; Borislav
> > Petkov <bp@...en8.de>; Weiny, Ira <ira.weiny@...el.com>; James Morse
> > <james.morse@....com>; Myron Stowe <myron.stowe@...hat.com>;
> > Andy Shevchenko <andriy.shevchenko@...ux.intel.com>; linux-
> > kernel@...r.kernel.org; linux-acpi@...r.kernel.org; linux-
> > nvdimm@...ts.01.org; Moore, Robert <robert.moore@...el.com>
> > Subject: [RFT][PATCH 2/3] ACPICA: Remove unused memory mappings on
> > interpreter exit
> >
> > From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> >
> > 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@...el.com>
> > ---
> >  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!