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

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Message-ID: <20200813232311.GJ4295@paulmck-ThinkPad-P72>
Date:   Thu, 13 Aug 2020 16:23:11 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     peterz@...radead.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 Fri, Aug 14, 2020 at 12:06:19AM +0200, peterz@...radead.org wrote:
> 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 ?

You are indeed not allowed to use kfree() under a raw spinlock, given
that it can acquire a non-raw spinlock.

But kfree_rcu() was just a wrapper around call_rcu(), which can be and
is called from raw atomic context.

> Can someone please spell out the actual problem?

And as noted above, reducing the kfree()-time cache misses would be
a good thing.

						Thanx, Paul