linux-kernel - Re: [PATCH 7/7] mm/page_alloc: Replace local

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Date:   Thu, 16 Jun 2022 15:07:25 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

On Thu, Jun 16, 2022 at 11:02 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/13/22 14:56, Mel Gorman wrote:
> > struct per_cpu_pages is no longer strictly local as PCP lists can be
> > drained remotely using a lock for protection. While the use of local_lock
> > works, it goes against the intent of local_lock which is for "pure
> > CPU local concurrency control mechanisms and not suited for inter-CPU
> > concurrency control" (Documentation/locking/locktypes.rst)
> >
> > local_lock protects against migration between when the percpu pointer is
> > accessed and the pcp->lock acquired. The lock acquisition is a preemption
> > point so in the worst case, a task could migrate to another NUMA node
> > and accidentally allocate remote memory. The main requirement is to pin
> > the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.
> >
> > Replace local_lock with helpers that pin a task to a CPU, lookup the
> > per-cpu structure and acquire the embedded lock. It's similar to local_lock
> > without breaking the intent behind the API. It is not a complete API
> > as only the parts needed for PCP-alloc are implemented but in theory,
> > the generic helpers could be promoted to a general API if there was
> > demand for an embedded lock within a per-cpu struct with a guarantee
> > that the per-cpu structure locked matches the running CPU and cannot use
> > get_cpu_var due to RT concerns. PCP requires these semantics to avoid
> > accidentally allocating remote memory.
> >
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
>
> ...
>
> > @@ -3367,30 +3429,17 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
> >       return min(READ_ONCE(pcp->batch) << 2, high);
> >  }
> >
> > -/* Returns true if the page was committed to the per-cpu list. */
> > -static bool free_unref_page_commit(struct page *page, int migratetype,
> > -                                unsigned int order, bool locked)
> > +static void free_unref_page_commit(struct per_cpu_pages *pcp, struct zone *zone,
> > +                                struct page *page, int migratetype,
> > +                                unsigned int order)
>
> Hmm given this drops the "bool locked" and bool return value again, my
> suggestion for patch 5/7 would result in less churn as those woudn't need to
> be introduced?
>
> ...
>
> > @@ -3794,19 +3805,29 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
> >       struct list_head *list;
> >       struct page *page;
> >       unsigned long flags;
> > +     unsigned long __maybe_unused UP_flags;
> >
> > -     local_lock_irqsave(&pagesets.lock, flags);
> > +     /*
> > +      * spin_trylock_irqsave is not necessary right now as it'll only be
> > +      * true when contending with a remote drain. It's in place as a
> > +      * preparation step before converting pcp locking to spin_trylock
> > +      * to protect against IRQ reentry.
> > +      */
> > +     pcp_trylock_prepare(UP_flags);
> > +     pcp = pcp_spin_trylock_irqsave(zone->per_cpu_pageset, flags);
> > +     if (!pcp)
>
> Besides the missing unpin Andrew fixed, I think also this is missing
> pcp_trylock_finish(UP_flags); ?

spin_trylock only fails when trylock_finish is a NOP.