Messages in this thread | | | From | Dan Williams <> | Date | Thu, 7 May 2020 14:20:30 -0700 | Subject | Re: [PATCH] ACPI: Drop rcu usage for MMIO mappings |
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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 > > 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@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.
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