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Message-ID: <bd3963f3-c6d6-f138-331c-9ac065633491@intel.com>
Date: Thu, 7 May 2020 18:43:03 +0200
From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: stable@...r.kernel.org, Len Brown <lenb@...nel.org>,
Borislav Petkov <bp@...en8.de>,
Ira Weiny <ira.weiny@...el.com>,
James Morse <james.morse@....com>,
Erik Kaneda <erik.kaneda@...el.com>,
Myron Stowe <myron.stowe@...hat.com>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: 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@...r.kernel.org>
> Fixes: 620242ae8c3d ("ACPI: Maintain a list of ACPI memory mapped I/O remappings")
> Cc: Len Brown <lenb@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: James Morse <james.morse@....com>
> Cc: Erik Kaneda <erik.kaneda@...el.com>
> Cc: Myron Stowe <myron.stowe@...hat.com>
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.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;
> }
>
>
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