linux-kernel - Re: Raw spinlocks and memory allocation

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Message-ID: <20200731204855.GR9247@paulmck-ThinkPad-P72>
Date:   Fri, 31 Jul 2020 13:48:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, hannes@...xchg.org, willy@...radead.org,
        urezki@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Raw spinlocks and memory allocation

On Fri, Jul 31, 2020 at 01:38:34PM -0700, Andrew Morton wrote:
> On Thu, 30 Jul 2020 16:12:05 -0700 "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > So, may we add a GFP_ flag that will cause kmalloc() and friends to return
> > NULL when they would otherwise need to acquire their non-raw spinlock?
> > This avoids adding any overhead to the slab-allocator fastpaths, but
> > allows callback invocation to reduce cache misses without having to
> > restructure some existing callers of call_rcu() and potential future
> > callers of kfree_rcu().
> 
> We have eight free gfp_t bits so that isn't a problem.

Whew!!!  ;-)

> Adding a test-n-branch to the kmalloc() fastpath may well be a concern.
> 
> Which of mm/sl?b.c are affected?

None of them, it turns out.  The initial patch will instead directly
invoke __get_free_page().  So we could just leave sl?b.c alone.

> A doesnt-need-to-really-work protopatch would help us understand the
> potential cost?

Makes sense!  My guess is that Uladzislau Rezki (CCed) will be sending
one along by the middle of next week.

							Thanx, Paul