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

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.

+static void __iomem *acpi_os_rw_map(acpi_physical_address phys_addr, + unsigned int size, bool *did_fallback) +{ + void __iomem *virt_addr = NULL;

+ if (WARN_ONCE(in_interrupt(), "ioremap in interrupt context\n")) + return NULL; + + /* Try to use a cached mapping and fallback otherwise */ + *did_fallback = false; + mutex_lock(&acpi_ioremap_lock); + virt_addr = acpi_map_vaddr_lookup(phys_addr, size); + if (virt_addr) + return virt_addr; + mutex_unlock(&acpi_ioremap_lock); + + virt_addr = acpi_os_ioremap(phys_addr, size); + *did_fallback = true; + + return virt_addr; +}

+static void acpi_os_rw_unmap(void __iomem *virt_addr, bool did_fallback) +{ + if (did_fallback) { + /* in the fallback case no lock is held */ + iounmap(virt_addr); + return; + } + + mutex_unlock(&acpi_ioremap_lock); +}