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

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.

---

drivers/acpi/osl.c | 117 +++++++++++++++++++++++++--------------------------- 1 file changed, 57 insertions(+), 60 deletions(-)

diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c index 762c5d50b8fe..207528c71e9c 100644 --- a/drivers/acpi/osl.c +++ b/drivers/acpi/osl.c @@ -214,13 +214,13 @@ acpi_physical_address __init acpi_os_get_root_pointer(void) return pa; } -/* Must be called with 'acpi_ioremap_lock' or RCU read lock held. */ static struct acpi_ioremap * acpi_map_lookup(acpi_physical_address phys, acpi_size size) { struct acpi_ioremap *map;

• list_for_each_entry_rcu(map, &acpi_ioremaps, list, acpi_ioremap_lock_held())

• lockdep_assert_held(&acpi_ioremap_lock);
• list_for_each_entry(map, &acpi_ioremaps, list) if (map->phys <= phys && phys + size <= map->phys + map->size) return map;

@@ -228,7 +228,6 @@ acpi_map_lookup(acpi_physical_address phys, acpi_size size) return NULL; } -/* Must be called with 'acpi_ioremap_lock' or RCU read lock held. */ static void __iomem * acpi_map_vaddr_lookup(acpi_physical_address phys, unsigned int size) { @@ -263,7 +262,8 @@ acpi_map_lookup_virt(void __iomem *virt, acpi_size size) { struct acpi_ioremap *map;

• list_for_each_entry_rcu(map, &acpi_ioremaps, list, acpi_ioremap_lock_held())

• lockdep_assert_held(&acpi_ioremap_lock);
• list_for_each_entry(map, &acpi_ioremaps, list) if (map->virt <= virt && virt + size <= map->virt + map->size) return map;

@@ -360,7 +360,7 @@ void __iomem __ref map->size = pg_sz; map->refcount = 1;

• list_add_tail_rcu(&map->list, &acpi_ioremaps);

• list_add_tail(&map->list, &acpi_ioremaps);

out: mutex_unlock(&acpi_ioremap_lock); @@ -374,20 +374,13 @@ void *__ref acpi_os_map_memory(acpi_physical_address phys, acpi_size size) } EXPORT_SYMBOL_GPL(acpi_os_map_memory); -/* Must be called with mutex_lock(&acpi_ioremap_lock) */ -static unsigned long acpi_os_drop_map_ref(struct acpi_ioremap *map) -{

• unsigned long refcount = --map->refcount;
• if (!refcount)

• list_del_rcu(&map->list);
  

• return refcount;

-}

-static void acpi_os_map_cleanup(struct acpi_ioremap *map) +static void acpi_os_drop_map_ref(struct acpi_ioremap *map) {

• synchronize_rcu_expedited();

• lockdep_assert_held(&acpi_ioremap_lock);
• if (--map->refcount > 0)

• return;
  

  acpi_unmap(map->phys, map->virt);
• list_del(&map->list); kfree(map); }

@@ -408,7 +401,6 @@ static void acpi_os_map_cleanup(struct acpi_ioremap *map) void __ref acpi_os_unmap_iomem(void __iomem *virt, acpi_size size) { struct acpi_ioremap *map;

• unsigned long refcount;

if (!acpi_permanent_mmap) { __acpi_unmap_table(virt, size); @@ -422,11 +414,8 @@ void __ref acpi_os_unmap_iomem(void __iomem *virt, acpi_size size) WARN(true, PREFIX "%s: bad address %p\n", __func__, virt); return; }

• refcount = acpi_os_drop_map_ref(map);

• acpi_os_drop_map_ref(map); mutex_unlock(&acpi_ioremap_lock);

• if (!refcount)

• acpi_os_map_cleanup(map);
  

  } EXPORT_SYMBOL_GPL(acpi_os_unmap_iomem);

@@ -461,7 +450,6 @@ void acpi_os_unmap_generic_address(struct acpi_generic_address *gas) { u64 addr; struct acpi_ioremap *map;

• unsigned long refcount;

if (gas->space_id != ACPI_ADR_SPACE_SYSTEM_MEMORY) return; @@ -477,11 +465,8 @@ void acpi_os_unmap_generic_address(struct acpi_generic_address *gas) mutex_unlock(&acpi_ioremap_lock); return; }

• refcount = acpi_os_drop_map_ref(map);

• acpi_os_drop_map_ref(map); mutex_unlock(&acpi_ioremap_lock);

• if (!refcount)

• acpi_os_map_cleanup(map);
  

  } EXPORT_SYMBOL(acpi_os_unmap_generic_address);

@@ -700,55 +685,71 @@ int acpi_os_read_iomem(void __iomem *virt_addr, u64 *value, u32 width) return 0; } +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);

+}

• acpi_status acpi_os_read_memory(acpi_physical_address phys_addr, u64 *value, u32 width) {

• void __iomem *virt_addr; unsigned int size = width / 8;
• bool unmap = false;

• bool did_fallback = false;
• void __iomem *virt_addr; u64 dummy; int error;

• rcu_read_lock();
• virt_addr = acpi_map_vaddr_lookup(phys_addr, size);
• if (!virt_addr) {

• rcu_read_unlock();
  

• virt_addr = acpi_os_ioremap(phys_addr, size);
  

• if (!virt_addr)
  

• 	return AE_BAD_ADDRESS;
  

• unmap = true;
  

• }

• virt_addr = acpi_os_rw_map(phys_addr, size, &did_fallback);
• if (!virt_addr)

• return AE_BAD_ADDRESS;
  

  if (!value) value = &dummy;

error = acpi_os_read_iomem(virt_addr, value, width); BUG_ON(error);

• if (unmap)

• iounmap(virt_addr);
  

• else

• rcu_read_unlock();
  

• acpi_os_rw_unmap(virt_addr, did_fallback); return AE_OK; }

acpi_status acpi_os_write_memory(acpi_physical_address phys_addr, u64 value, u32 width) {

• void __iomem *virt_addr; unsigned int size = width / 8;
• bool unmap = false;

• bool did_fallback = false;
• void __iomem *virt_addr;

• rcu_read_lock();
• virt_addr = acpi_map_vaddr_lookup(phys_addr, size);
• if (!virt_addr) {

• rcu_read_unlock();
  

• virt_addr = acpi_os_ioremap(phys_addr, size);
  

• if (!virt_addr)
  

• 	return AE_BAD_ADDRESS;
  

• unmap = true;
  

• }

• virt_addr = acpi_os_rw_map(phys_addr, size, &did_fallback);
• if (!virt_addr)

• return AE_BAD_ADDRESS;
  

switch (width) { case 8: @@ -767,11 +768,7 @@ acpi_os_write_memory(acpi_physical_address phys_addr, u64 value, u32 width) BUG(); }

• if (unmap)

• iounmap(virt_addr);
  

• else

• rcu_read_unlock();
  

• acpi_os_rw_unmap(virt_addr, did_fallback); return AE_OK; }