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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Thu, 1 Oct 2020 21:32:16 +0200
To:     Michal Hocko <mhocko@suse.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Message-ID: <20201001193216.GB29606@pc636>
References: <20200918194817.48921-1-urezki@gmail.com>
 <20200918194817.48921-3-urezki@gmail.com>
 <38f42ca1-ffcd-04a6-bf11-618deffa897a@suse.cz>
 <20200929220742.GB8768@pc636>
 <20200930103557.GQ2277@dhcp22.suse.cz>
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On Wed, Sep 30, 2020 at 12:35:57PM +0200, Michal Hocko wrote:
> On Wed 30-09-20 00:07:42, Uladzislau Rezki wrote:
> [...]
> > <snip>
> > bool is_pcp_cache_empty(gfp_t gfp)
> > {
> >     struct per_cpu_pages *pcp;
> >     struct zoneref *ref;
> >     unsigned long flags;
> >     bool empty;
> > 
> >     ref = first_zones_zonelist(node_zonelist(
> >             numa_node_id(), gfp), gfp_zone(gfp), NULL);
> >     if (!ref->zone)
> >             return true;
> > 
> >     local_irq_save(flags);
> >     pcp = &this_cpu_ptr(ref->zone->pageset)->pcp;
> >     empty = list_empty(&pcp->lists[gfp_migratetype(gfp)]);
> >     local_irq_restore(flags);
> > 
> >     return empty;
> > }
> > 
> > disable_irq();
> > if (!is_pcp_cache_empty(GFP_NOWAIT))
> >     __get_free_page(GFP_NOWAIT);
> > enable_irq();
> > <snip>
> > 
> > Do you mean to have something like above? I mean some extra API
> > function that returns true or false if fast-fast allocation can
> > either occur or not. Above code works just fine and never touches
> > main zone->lock.
> 
> The above code works with the _current_ implementation and it restricts
> its implementation to some degree. Future changes might get harder to
> implement with a pattern like this. I do not think we want users to be
> aware of internal implementation details like pcp caches, migrate types
> or others. While pcp caches are here for years and unlikely to change in
> a foreseeable future many details are changing on regular basis.
>
I see your view. That was en example for better understanding.

Thanks.

--
Vlad Rezki