On 7/3/19 10:37 AM, Michal Hocko wrote:

On Wed 03-07-19 09:12:13, Waiman Long wrote:

quoted

On 7/3/19 2:56 AM, Michal Hocko wrote:

quoted

On Tue 02-07-19 14:37:30, Waiman Long wrote:

quoted

Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
file to shrink the slab by flushing all the per-cpu slabs and free
slabs in partial lists. This applies only to the root caches, though.

Extends this capability by shrinking all the child memcg caches and
the root cache when a value of '2' is written to the shrink sysfs file.

Why do we need a new value for this functionality? I would tend to think
that skipping memcg caches is a bug/incomplete implementation. Or is it
a deliberate decision to cover root caches only?

It is just that I don't want to change the existing behavior of the
current code. It will definitely take longer to shrink both the root
cache and the memcg caches.

Does that matter? To whom and why? I do not expect this interface to be
used heavily.

The only concern that I can see is the fact that I need to take the
slab_mutex when iterating the memcg list to prevent concurrent
modification. That may have some impact on other applications running in
the system. However, I can put a precaution statement on the user-doc to
discuss the potential performance impact.

quoted

If we all agree that the only sensible
operation is to shrink root cache and the memcg caches together. I am
fine just adding memcg shrink without changing the sysfs interface
definition and be done with it.

The existing documentation is really modest on the actual semantic:
Description:
                The shrink file is written when memory should be reclaimed from
                a cache.  Empty partial slabs are freed and the partial list is
                sorted so the slabs with the fewest available objects are used
                first.

which to me sounds like all slabs are free and nobody should be really
thinking of memcgs. This is simply drop_caches kinda thing. We surely do
not want to drop caches only for the root memcg for /proc/sys/vm/drop_caches
right?

I am planning to reword the document to make the effect of using this
sysfs file more explicit.

Cheers,
Longman