linux-kernel - Re: [PATCH 6/7] rcu/tiny: support reclaim for head-less object

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Message-ID: <20200330144227.GA16651@pc636>
Date:   Mon, 30 Mar 2020 16:42:27 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        RCU <rcu@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 6/7] rcu/tiny: support reclaim for head-less object

> 
> Hmm, Ok. So on -tiny, if there's any allocation failure ever, we immediately
> revert to call_rcu(). I guess we could also create a regular (non-array)
> queue for objects with an rcu_head and queue it on that (since it does not
> need allocation) in case of array allocation failure, however that may not be
> worth it. So this LGTM. Thanks!
> 
> For entire series:
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> (I will submit a follow-up to fix the tagging, please let me submit Vlad's
> entire series with some patches on top -- I also did a bit of wordsmithing in
> the commit messages of this series).
>
Thank you, Joel, for your review and help!

> 
> Loved the might_sleep() idea btw, I suppose if atomic context wants to do
> kvfree_rcu(), then we could also have kfree_rcu() defer the kvfree_rcu() to
> execute from a workqueue. Thoughts? We can then allow poor insomniacs from
> calling this API :)
> 
Not sure if i understand you correctly. Could you please <snip> some code
for illustration?

As far as i understand, it should be done then synchronously. We can defer  
and queue some work to do it in worqueue context. But i am not sure how
to proccess next coming request, i.e. busy waiting until we manage to push
a new ptr. to free? But in that case it would not work if there is only
one CPU available.

Thanks!

--
Vlad Rezki