Re: [PATCH] ACPI: Drop rcu usage for MMIO mappings

[Dan Williams]

On Thu, May 7, 2020 at 9:43 AM Rafael J. Wysocki
<rafael.j.wysocki@xxxxxxxxx> wrote:
>
> On 5/7/2020 1:39 AM, Dan Williams wrote:
> > Recently a performance problem was reported for a process invoking a
> > non-trival ASL program. The method call in this case ends up
> > repetitively triggering a call path like:
> >
> >      acpi_ex_store
> >      acpi_ex_store_object_to_node
> >      acpi_ex_write_data_to_field
> >      acpi_ex_insert_into_field
> >      acpi_ex_write_with_update_rule
> >      acpi_ex_field_datum_io
> >      acpi_ex_access_region
> >      acpi_ev_address_space_dispatch
> >      acpi_ex_system_memory_space_handler
> >      acpi_os_map_cleanup.part.14
> >      _synchronize_rcu_expedited.constprop.89
> >      schedule
> >
> > The end result of frequent synchronize_rcu_expedited() invocation is
> > tiny sub-millisecond spurts of execution where the scheduler freely
> > migrates this apparently sleepy task. The overhead of frequent scheduler
> > invocation multiplies the execution time by a factor of 2-3X.
> >
> > For example, performance improves from 16 minutes to 7 minutes for a
> > firmware update procedure across 24 devices.
> >
> > Perhaps the rcu usage was intended to allow for not taking a sleeping
> > lock in the acpi_os_{read,write}_memory() path which ostensibly could be
> > called from an APEI NMI error interrupt? Neither rcu_read_lock() nor
> > ioremap() are interrupt safe, so add a WARN_ONCE() to validate that rcu
> > was not serving as a mechanism to avoid direct calls to ioremap(). Even
> > the original implementation had a spin_lock_irqsave(), but that is not
> > NMI safe.
> >
> > APEI itself already has some concept of avoiding ioremap() from
> > interrupt context (see erst_exec_move_data()), if the new warning
> > triggers it means that APEI either needs more instrumentation like that
> > to pre-emptively fail, or more infrastructure to arrange for pre-mapping
> > the resources it needs in NMI context.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O remappings")
> > Cc: Len Brown <lenb@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> > Cc: James Morse <james.morse@xxxxxxx>
> > Cc: Erik Kaneda <erik.kaneda@xxxxxxxxx>
> > Cc: Myron Stowe <myron.stowe@xxxxxxxxxx>
> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>
> linux-acpi is kind of relevant for this too, so please CC it.

Whoops, my bad. Will resend with some of Andy's cleanup suggestions.