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

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Date:   Thu, 13 Aug 2020 18:14:32 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Matthew Wilcox <willy@...radead.org>
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

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.

Thanks,

        tglx