[PATCH v2] ACPI: Drop rcu usage for MMIO mappings

[Dan Williams]

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@xxxxxxxxxxxxxxx>
Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O remappings")
Cc: Len Brown <lenb@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: James Morse <james.morse@xxxxxxx>
Cc: Erik Kaneda <erik.kaneda@xxxxxxxxx>
Cc: Myron Stowe <myron.stowe@xxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Changes since v1 [1]:

- Actually cc: the most important list for ACPI changes (Rafael)

- Cleanup unnecessary variable initialization (Andy)

Link: https://lore.kernel.org/linux-nvdimm/158880834905.2183490.15616329469420234017.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/


 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..a44b75aac5d0 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;
+
+	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;
 }