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    SubjectRe: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
    On Fri, Sep 25, 2020 at 05:17:12PM +0100, Mel Gorman wrote:
    > On Fri, Sep 25, 2020 at 05:31:29PM +0200, Uladzislau Rezki wrote:
    > > > > > >
    > > > > > > All good points!
    > > > > > >
    > > > > > > On the other hand, duplicating a portion of the allocator functionality
    > > > > > > within RCU increases the amount of reserved memory, and needlessly most
    > > > > > > of the time.
    > > > > > >
    > > > > >
    > > > > > But it's very similar to what mempools are for.
    > > > > >
    > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU
    > > > > to move things forward and it might be not efficient way. As a side
    > > > > effect, maintaining of the bulk arrays in the separate worker thread
    > > > > will introduce other drawbacks:
    > > >
    > > > This is true but it is also true that it is RCU to require this special
    > > > logic and we can expect that we might need to fine tune this logic
    > > > depending on the RCU usage. We definitely do not want to tune the
    > > > generic page allocator for a very specific usecase, do we?
    > > >
    > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
    > > to provide a memory service for contexts which are not allowed to
    > > sleep, RCU is part of them. Both flags used to provide such ability
    > > before but not anymore.
    > >
    > > Do you agree with it?
    > >
    >
    > I was led to believe that hte problem was taking the zone lock while
    > holding a raw spinlock that was specific to RCU.
    In RCU code we hold a raw spinlock, because the kfree_rcu() should
    follow the call_rcu() rule and work in atomic contexts. So we can
    not enter a page allocator because it uses spinlock_t z->lock(is sleepable for RT).

    Because of CONFIG_PROVE_RAW_LOCK_NESTING option and CONFIG_PREEMPT_RT.

    >
    > Are you saying that GFP_ATOMIC/GFP_NOWAIT users are also holding raw
    > spinlocks at the same time on RT?
    >
    I do not say it. And it is not possible because zone->lock has
    a spinlock_t type. So, in case of CONFIG_PREEMPT_RT you will
    hit a "BUG: scheduling while atomic". If allocator is called
    when: raw lock is held or irqs are disabled or preempt_disable()
    on a higher level.

    --
    Vlad Rezki

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