Re: [PATCH v3 0/2] arch_topology: Pre-allocate cacheinfo from primary CPU

[Pierre Gondois]

Hello Radu,
Some additional points:
1-
For the record, Will made a comment about adding weak functions
(cf. https://lore.kernel.org/all/20230327121734.GB31342@willie-the-truck/)
but I don't see how it could be done otherwise ...

2-
The patch-set needs to be rebased on top of v6.3-rc6,
otherwise there is a merge conflict.

3-
When trying the patch-set on an ACPI platform with no PPTT, it seems that
fetch_cache_info() is not called from init_cpu_topology() because
parse_acpi_topology() returns an error code. This result in a
'sleeping function called from invalid context' message. The following made
it work for me:

--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -838,7 +838,6 @@ void __init init_cpu_topology(void)
                 * don't use partial information.
                 */
                reset_cpu_topology();
-               return;
        }
 
        for_each_possible_cpu(cpu) {

With 2 and 3 addressed:
Reviewed-by: Pierre Gondois <pierre.gondois@xxxxxxx>

Also maybe wait for Sudeep to have a look before sending a v4,
Regards,
Pierre


On 4/7/23 01:39, Radu Rendec wrote:
> Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
> tries to build the cacheinfo from the primary CPU prior to secondary
> CPUs boot, if the DT/ACPI description contains cache information.
> However, if such information is not present, it still reverts to the old
> behavior, which allocates the cacheinfo memory on each secondary CPU. On
> RT kernels, this triggers a "BUG: sleeping function called from invalid
> context" because the allocation is done before preemption is first
> enabled on the secondary CPU.
>
> The solution is to add cache information to DT/ACPI, but at least on
> arm64 systems this can be avoided by leveraging automatic detection
> (through the CLIDR_EL1 register), which is already implemented but
> currently doesn't work on RT kernels for the reason described above.
>
> This patch series attempts to enable automatic detection for RT kernels
> when no DT/ACPI cache information is available, by pre-allocating
> cacheinfo memory on the primary CPU.
>
> The first patch adds an architecture independent infrastructure that
> allows architecture specific code to take an early guess at the number
> of cache leaves of the secodary CPUs, while it runs in preemptible
> context on the primary CPU. At the same time, it gives architecture
> specific code the opportunity to go back later, while it runs on the
> secondary CPU, and reallocate the cacheinfo memory if the initial guess
> proves to be wrong.
>
> The second patch leverages the infrastructure implemented in the first
> patch and enables early cache depth detection for arm64.
>
> The patch series is based on an RFC patch that was posted to the
> linux-arm-kernel mailing list and discussed with a smaller audience:
> https://lore.kernel.org/all/20230323224242.31142-1-rrendec@xxxxxxxxxx/
>
> Changes to v2:
> * Address minor coding style issue (unbalanced braces).
> * Move cacheinfo reallocation logic from detect_cache_attributes() to a
>    new function to improve code readability.
> * Minor fix to cacheinfo reallocation logic to avoid a new detection of
>    the cache level if/when detect_cache_attributes() is called again.
>
> Radu Rendec (2):
>    cacheinfo: Add arch specific early level initializer
>    cacheinfo: Add arm64 early level initializer implementation
>
>   arch/arm64/kernel/cacheinfo.c | 32 +++++++++++----
>   drivers/base/cacheinfo.c      | 75 +++++++++++++++++++++++++----------
>   include/linux/cacheinfo.h     |  2 +
>   3 files changed, 79 insertions(+), 30 deletions(-)