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

[Dan Williams]

On Thu, May 7, 2020 at 2:25 AM Andy Shevchenko
<andy.shevchenko@xxxxxxxxx> wrote:
>
> On Thu, May 7, 2020 at 3:21 AM Dan Williams <dan.j.williams@xxxxxxxxx> 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.
>
> ...
>
> > +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.

True, holdover from a previous version, will drop.

>
> > +       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...

Seems like it is:

$ sparse --version
v0.6.1-191-gc51a0382202e

$ cat out | grep osl\.c
  CHECK   drivers/acpi/osl.c
drivers/acpi/osl.c:373:17: warning: cast removes address space
'<asn:2>' of expression

...was the only warning I got.