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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 31 Mar 2020 19:02:32 +0200
To:     Uladzislau Rezki <urezki@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org,
        neilb@suse.com, vbabka@suse.cz, mgorman@suse.de,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free
 memory pattern
Message-ID: <20200331170232.GA28413@pc636>
References: <20200331131628.153118-1-joel@joelfernandes.org>
 <20200331140433.GA26498@pc636>
 <20200331150911.GC236678@google.com>
 <20200331160119.GA27614@pc636>
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> >
> > Paul was concerned about following scenario with hitting synchronize_rcu():
> > 1. Consider a system under memory pressure.
> > 2. Consider some other subsystem X depending on another system Y which uses
> >    kfree_rcu(). If Y doesn't complete the operation in time, X accumulates
> >    more memory.
> > 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down.
> >    This causes X to further allocate memory, further causing a chain
> >    reaction.
> > Paul, please correct me if I'm wrong.
> > 
> I see your point and agree that in theory it can happen. So, we should
> make it more tight when it comes to rcu_head attachment logic.
> 
Just adding more thoughts about such concern. Even though in theory we
can run into something like that. But also please note, that under high
memory pressure it also does not mean that (X) will always succeed with
further infinite allocations, so memory pressure is something common.
As soon as the situation becomes slightly better we do our work much
efficient.

Practically, i was trying to simulate memory pressure to hit synchronize_rcu()
on my test system. By just simulating head-less freeing(for any object) and
by always dynamic attaching path. So i could trigger it, but that was really
hard to achieve and it happened only few times. So that was not like a constant
hit. What i got constantly were:

- System got recovered and proceed with "normal" path;
- The OOM hit as a final step, when the system is run out of memory fully.

So, practically i have not seen massive synchronize_rcu() hit.

--
Vlad Rezki