Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

From: Thomas Gleixner
Date: Thu Aug 13 2020 - 12:14:39 EST

Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.