Re: [RFC PATCH] ext4: cacheline-align inode cache to avoid false sharing

[JonasZhou-oc <jonaszhou-oc@zhaoxin.com>]

From: JonasZhou-oc <jonaszhou-oc@zhaoxin.com>

Thanks for pointing this out.

Yes, SLAB_HWCACHE_ALIGN aligns the allocation, but does not guarantee
that the embedded i_rwsem starts at a cache-line boundary in every
configuration. The offset of 384 bytes and the performance results
in the changelog apply only to the tested build. This dependency is
why I raised the layout question in the RFC.

In configurations with a different layout, the patch could increase
the allocation size without separating i_rwsem from the preceding
metadata. The size increase itself is also configuration-dependent;
the reported 16-byte increase applies to the tested build.

One possible refinement is to request SLAB_HWCACHE_ALIGN only when:

    IS_ALIGNED(offsetof(struct ext4_inode_info, vfs_inode) +
               offsetof(struct inode, i_rwsem),
               cache_line_size())

This would restrict the change to layouts where aligning the object
also places i_rwsem at a cache-line boundary. It would separate the
lock from preceding fields, but would not guarantee a performance
improvement or eliminate possible sharing with fields following it.

I have not yet tested this conditional variant. It would still need
configuration coverage and workload validation, including checking
the actual slab memory cost.

Would this layout-dependent opt-in be preferable, or would maintainers
prefer evaluating unconditional alignment with broader performance
and memory measurements?

Thanks,
Jonas