linux-kernel - Re: [PATCH RFC] rcu/tree: Refactor object allocation and try harder for array allocation

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Message-ID: <20200424122835.GA4686@pc636>
Date:   Fri, 24 Apr 2020 14:28:35 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Refactor object allocation and try harder
 for array allocation

> 
> I think Johannes said that waking up kswapd is Ok. OTOH, I did not see
> the drawback in waking up kswapd to do background reclaim since it
> does not happen synchronously right? I think Johannes said we can do
> better than just waking kswapd by also doing light direct reclaim
> using __GFP_NORETRY but let me know if I missed something.
> 
Then i misunderstood that point. So, seems it is settled now. We just
use GFP_NOWAIT | __GFP_RECLAIM | __GFP_NORETRY | __GFP_NOWARN for headless
case, i.e. when we can sleep. It will do direct reclaim(slow path), but
light one because of __GFP_NORETRY.

Does it sound good?

> > For single argument we inline freeing into current context after
> > synchronize_rcu() because it follows might_sleep() annotation.
> 
> Yes.
> 
> Also, with the additional caching being planned, we could avoid the
> chances of hitting the synchronize_rcu inlining.
> 
Or minimize it.

There is also one question i would like to clarify. That is dynamic head
attaching that requires small allocations. Do we drop it?

Thanks!

--
Vlad Rezki