On Thu, May 7, 2020 at 9:43 AM Rafael J. Wysocki rafael.j.wysocki@intel.com 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 scheduleThe 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@vger.kernel.org Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O remappings") Cc: Len Brown lenb@kernel.org Cc: Borislav Petkov bp@alien8.de Cc: Ira Weiny ira.weiny@intel.com Cc: James Morse james.morse@arm.com Cc: Erik Kaneda erik.kaneda@intel.com Cc: Myron Stowe myron.stowe@redhat.com Cc: "Rafael J. Wysocki" rjw@rjwysocki.net Cc: Andy Shevchenko andriy.shevchenko@linux.intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com
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.