On Thu, May 7, 2020 at 9:43 AM Rafael J. Wysocki <[email protected]> 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: <[email protected]> > > Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O > > remappings") > > Cc: Len Brown <[email protected]> > > Cc: Borislav Petkov <[email protected]> > > Cc: Ira Weiny <[email protected]> > > Cc: James Morse <[email protected]> > > Cc: Erik Kaneda <[email protected]> > > Cc: Myron Stowe <[email protected]> > > Cc: "Rafael J. Wysocki" <[email protected]> > > Cc: Andy Shevchenko <[email protected]> > > Signed-off-by: Dan Williams <[email protected]> > > 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.
