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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 5 Jun 2020 19:02:24 +0200
Message-ID: <CAJZ5v0iAwDpFYgOa+1=7_wdt4jHVftzS-o=xTJzJSVjceqhzwQ@mail.gmail.com>
Subject: Re: [PATCH v2] ACPI: Drop rcu usage for MMIO mappings
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Stable <stable@vger.kernel.org>, Len Brown <lenb@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Ira Weiny <ira.weiny@intel.com>,
        James Morse <james.morse@arm.com>,
        Erik Kaneda <erik.kaneda@intel.com>,
        Myron Stowe <myron.stowe@redhat.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
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On Fri, Jun 5, 2020 at 6:39 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Fri, Jun 5, 2020 at 9:22 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> [..]
> > > The fix we are looking at now is to pre-map operation regions in a
> > > similar manner as the way APEI resources are pre-mapped. The
> > > pre-mapping would arrange for synchronize_rcu_expedited() to be elided
> > > on each dynamic mapping attempt. The other piece is to arrange for
> > > operation-regions to be mapped at their full size at once rather than
> > > a page at a time.
> >
> > However, if the RCU usage in ACPI OSL can be replaced with an rwlock,
> > some of the ACPICA changes above may not be necessary anymore (even
> > though some of them may still be worth making).
>
> I don't think you can replace the RCU usage in ACPI OSL and still
> maintain NMI lookups in a dynamic list.

I'm not sure what NMI lookups have to do with the issue at hand.

If acpi_os_{read|write}_memory() is used from NMI, that is a bug
already in there which is unrelated to the performance problem with
opregions.

> However, there are 3 solutions I see:
>
> - Prevent acpi_os_map_cleanup() from triggering at high frequency by
> pre-mapping and never unmapping operation-regions resources (internal
> discussion in progress)

Yes, that can be done, if necessary.

> - Prevent walks of the 'acpi_ioremaps' list (acpi_map_lookup_virt())
> from NMI context by re-writing the physical addresses in the APEI
> tables with pre-mapped virtual address, i.e. remove rcu_read_lock()
> and list_for_each_entry_rcu() from NMI context.

That sounds a bit convoluted to me.

> - Split operation-region resources into a separate mapping mechanism
> than APEI resources so that typical locking can be used for the
> sleepable resources and let the NMI accessible resources be managed
> separately.
>
> That last one is one we have not discussed internally, but it occurred
> to me when you mentioned replacing RCU.

So NMI cannot use acpi_os_{read|write}_memory() safely which you have
pointed out for a few times.

But even if NMI resources are managed separately, the others will
still not be sleepable (at least not all of them).

Cheers!