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SubjectRe: [PATCH RFC v2 02/10] mm: Make shrink_slab() lockless
On Wed 08-08-18 16:20:54, Kirill Tkhai wrote:
> [Added two more places needed srcu_dereference(). All ->shrinker_map
> dereferences must be under SRCU, and this v2 adds missed in previous]
> The patch makes shrinker list and shrinker_idr SRCU-safe
> for readers. This requires synchronize_srcu() on finalize
> stage unregistering stage, which waits till all parallel
> shrink_slab() are finished
> Note, that patch removes rwsem_is_contended() checks from
> the code, and this does not result in delays during
> registration, since there is no waiting at all. Unregistration
> case may be optimized by splitting unregister_shrinker()
> in tho stages, and this is made in next patches.
> Also, keep in mind, that in case of SRCU is not allowed
> to make unconditional (which is done in previous patch),
> it is possible to use percpu_rw_semaphore instead of it.
> percpu_down_read() will be used in shrink_slab_memcg()
> and in shrink_slab(), and consecutive calls
> percpu_down_write(percpu_rwsem);
> percpu_up_write(percpu_rwsem);
> will be used instead of synchronize_srcu().

An obvious question. Why didn't you go that way? What are pros/cons of
both approaches?
Michal Hocko

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