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

[Radu Rendec]

Hello Pierre,

On Tue, 2023-04-11 at 14:36 +0200, Pierre Gondois wrote:
> 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 ...

In that comment, Will suggested using static inline functions in a
header. It would probably work but for the sake of consistency with
init_cache_level() I would argue it's better to use a weak function in
this particular case.

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

Fair enough. I worked off of linux-rt-devel for obvious reasons, and
forgot to rebase. It's a trivial conflict, which both "git rebase" and
"git am -3" can fix automatically. I will keep this in mind for v4.

> 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) {

Good catch! I think this calls for a dedicated patch in the series to
do just that and explain why the "return" statement is being removed.

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

Thanks again for reviewing these patches and for all your input!

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

Sure. Looking at other patches, he seems to respond pretty quickly, so
I'll wait until tomorrow and then send v4 if I don't hear back.

Best regards,
Radu

> 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(-)
> > 
>