linux-kernel - Re: [PATCH 2/4] signal: Make flush_sigqueue() use free

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Date:   Fri, 22 Mar 2019 19:39:31 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Waiman Long <longman@...hat.com>
cc:     Oleg Nesterov <oleg@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release
 memory

On Fri, 22 Mar 2019, Waiman Long wrote:

> >
> >> I am looking forward to it.
> > There is also alrady rcu being used in these paths. kfree_rcu() would not
> > be enough? It is an estalished mechanism that is mature and well
> > understood.
> >
> In this case, the memory objects are from kmem caches, so they can't
> freed using kfree_rcu().

Oh they can. kfree() can free memory from any slab cache.

> There are certainly overhead using the kfree_rcu(), or a
> kfree_rcu()-like mechanism. Also I think the actual freeing is done at
> SoftIRQ context which can be a problem if there are too many memory
> objects to free.

No there is none that I am aware of. And this has survived testing of HPC
loads with gazillion of objects that have to be freed from multiple
processors. We really should not rearchitect this stuff... It took us
quite a long time to have this scale well under all loads.

Please use rcu_free().