Re: [PATCH] mm: percpu: Increase PERCPU_MODULE_RESERVE

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-06-11 07:45:46 [-1000], Tejun Heo wrote:
> 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.

I could avoid initialising per-CPU locks because of the static build
time init and avoid the alloc_percpu() at module init. I can access
members of the struct avoid one pointer dereference. All this for 576
bytes.

> Thanks.

Sebastian