On Thu, May 7, 2020 at 3:21 AM Dan Williams dan.j.williams@intel.com 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.
...
+static void __iomem *acpi_os_rw_map(acpi_physical_address phys_addr,
unsigned int size, bool *did_fallback)+{
void __iomem *virt_addr = NULL;
Assignment is not needed as far as I can see.
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;+}
I'm wondering if Sparse is okay with this...
+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);+}
...and this functions from locking perspective.