linux-kernel - Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

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Date:   Mon, 25 Jan 2021 16:39:43 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

On Mon 25-01-21 15:31:50, Uladzislau Rezki wrote:
> > On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote:
> > > For a single argument we can directly request a page from a caller
> > > context when a "carry page block" is run out of free spots. Instead
> > > of hitting a slow path we can request an extra page by demand and
> > > proceed with a fast path.
> > > 
> > > A single-argument kvfree_rcu() must be invoked in sleepable contexts,
> > > and that its fallback is the relatively high latency synchronize_rcu().
> > > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL
> > > to allow limited sleeping within the memory allocator.
> > 
> > __GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy
> > way to allocate without triggering the OOM killer. Is this really what
> > you need/want? Is __GFP_NORETRY too weak?
> > 
> Hm... We agreed to proceed with limited lightwait memory direct reclaim.
> Johannes Weiner proposed to go with __GFP_NORETRY flag as a starting
> point: https://www.spinics.net/lists/rcu/msg02856.html
> 
> <snip>
>     So I'm inclined to suggest __GFP_NORETRY as a starting point, and make
>     further decisions based on instrumentation of the success rates of
>     these opportunistic allocations.
> <snip>

I completely agree with Johannes here.

> but for some reason, i can't find a tail or head of it, we introduced
> __GFP_RETRY_MAYFAIL what is a heavy one from a time consuming point of view.
> What we would like to avoid.

Not that I object to this use but I think it would be much better to use
it based on actual data. Going along with it right away might become a
future burden to make any changes in this aspect later on due to lack of 
exact reasoning. General rule of thumb for __GFP_RETRY_MAYFAIL is really
try as hard as it can get without being really disruptive (like OOM
killing something). And your wording didn't really give me that
impression.

-- 
Michal Hocko
SUSE Labs