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

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Date:   Thu, 13 Aug 2020 17:22:13 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        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>,
        "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 06:14:32PM +0200, Thomas Gleixner wrote:
> Matthew Wilcox <willy@...radead.org> writes:
> > On Thu, Aug 13, 2020 at 03:27:15PM +0200, Thomas Gleixner wrote:
> >> And guarding it with RT is not working either because then you are back
> >> to square one with the problem which triggered the discussion in the
> >> first place:
> >> 
> >> raw_spin_lock()
> >>   alloc()
> >>     if (RT && !preemptible())  <- False because RT == false
> >>     	goto bail;
> >> 
> >>     spin_lock(&zone->lock)  --> LOCKDEP complains
> >> 
> >> So either you convince Paul not to do that or you need to do something
> >> like I suggested in my other reply.
> >
> > I'd like to throw in the possibility that we do something like:
> >
> >   raw_spin_lock()
> >     alloc()
> >       if (!spin_trylock(&zone->lock))
> >         if (RT && !preemptible())
> >           goto bail;
> >         spin_lock(&zone->lock);
> >
> > would that make us feel more comfortable about converting zone->lock to
> > a raw spinlock?
> 
> Even if that could cure that particular problem of allocations in deep
> atomic context, making zone->lock raw brings back the problem of
> zone->lock being held/contended for hundreds of microseconds with
> interrupts disabled which is causing RT tasks to miss their deadlines by
> big margins.

Ah, I see.  Yeah, that doesn't work.  Never mind.