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Message-ID: <20200811153327.GW4295@paulmck-ThinkPad-P72>
Date: Tue, 11 Aug 2020 08:33:27 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: 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 Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote:
> Michal Hocko <mhocko@...e.com> writes:
> > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> >> > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote:
> >> > Is there any fundamental problem to make zone raw_spin_lock?
> >> >
> >> Good point. Converting a regular spinlock to the raw_* variant can solve
> >> an issue and to me it seems partly reasonable. Because there are other
> >> questions if we do it:
> >>
> >> a) what to do with kswapd and "wake-up path" that uses sleepable lock:
> >> wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait).
> >
> > If there is no RT friendly variant for waking up process from the atomic
> > context then we might need to special case this for the RT tree.
>
> That's a solvable problem.
>
> >> b) How RT people reacts on it? I guess they will no be happy.
> >
> > zone->lock should be held for a very limited amount of time.
>
> Emphasis on should. free_pcppages_bulk() can hold it for quite some time
> when a large amount of pages are purged. We surely would have converted
> it to a raw lock long time ago otherwise.
>
> For regular enterprise stuff a few hundred microseconds might qualify as
> a limited amount of time. For advanced RT applications that's way beyond
> tolerable..
>
> >> As i described before, calling the __get_free_page(0) with 0 as argument
> >> will solve the (a). How correctly is it? From my point of view the logic
> >> that bypass the wakeup path should be explicitly defined.
> >
> > gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which
> > doesn't wake up kswapd. So if the wakeup is a problem then this would be
> > a way to go.
>
> The wakeup is the least of my worries.
>
> > To me it would make more sense to support atomic allocations also for
> > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> > work for atomic context in RT sounds subtle and wrong.
>
> Well, no. RT moves almost everything out of atomic context which means
> that GFP_ATOMIC is pretty meanlingless on a RT kernel. RT sacrifies
> performance for determinism. It's a known tradeoff.
>
> Now RCU creates a new thing which enforces to make page allocation in
> atomic context possible on RT. What for?
>
> What's the actual use case in truly atomic context for this new thing on
> an RT kernel?
It is not just RT kernels. CONFIG_PROVE_RAW_LOCK_NESTING=y propagates
this constraint to all configurations, and a patch in your new favorite
subsystem really did trigger this lockdep check in a non-RT kernel.
> The actual RCU code disabling interrupts is an implementation detail
> which can easily be mitigated with a local lock.
In this case, we are in raw-spinlock context on entry to kfree_rcu().
Thanx, Paul
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