Re: [PATCH] mm: percpu: Increase PERCPU_MODULE_RESERVE

[Tejun Heo <tj@kernel.org>]

On Wed, Jun 11, 2025 at 06:18:49PM +0200, Sebastian Andrzej Siewior wrote:
> PERCPU_MODULE_RESERVE defines the maximum size that can by used for the
> per-CPU data size used by modules. This is 8KiB.
> 
> Commit 035fcdc4d240c ("openvswitch: Merge three per-CPU structures into
> one") restructured the per-CPU memory allocation for openvswitch and
> moved the separate alloc_percpu() invocations at module init time to a
> static per-CPU variable which is allocated by the module loader.
> 
> The size of the per-CPU data section for openvswitch is 6488 bytes which
> is ~80% of the available per-CPU memory. Together with a few other
> modules it is easy to exhaust the available 8KiB of memory.
> 
> The memory range for the per-CPU memory is allocated early and pages for
> its backing are only allocated once the per-CPU memory is allocated.
> Increasing the map from 8 to 16 KiB adds 256 bytes to the alloc_map and
> bound_map and 64 bytes to md_blocks (576 bytes in total).
> 
> Increase the available memory for module's per-CPU data section to
> 16KiB.

I think a better direction would be keeping using alloc_percpu(). There
aren't a lot of benefits to using static definitions compared to dynamic
ones and making it larger increases overhead for everyone.

Thanks.

-- 
tejun