linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Fri, 14 Aug 2020 00:06:19 +0200
From:   peterz@...radead.org
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Thu, Aug 13, 2020 at 11:52:57AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 13, 2020 at 08:26:18PM +0200, peterz@...radead.org wrote:

> > I thought the rule was:
> > 
> >  - No allocators (alloc/free) inside raw_spinlock_t, full-stop.
> > 
> > Why are we trying to craft an exception?
> 
> So that we can reduce post-grace-period cache misses by a factor of
> eight when invoking RCU callbacks.  This reduction in cache misses also
> makes it more difficult to overrun RCU with floods of either call_rcu()
> or kfree_rcu() invocations.
> 
> The idea is to allocate page-sized arrays of pointers so that the callback
> invocation can sequence through the array instead of walking a linked
> list, hence the reduction in cache misses.

I'm still not getting it, how do we end up trying to allocate memory
from under raw spinlocks if you're not allowed to use kfree_rcu() under
one ?

Can someone please spell out the actual problem?