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References: <20201116220033.1837-1-urezki@gmail.com> <20201116220033.1837-2-urezki@gmail.com>
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From:   huang ying <huang.ying.caritas@gmail.com>
Date:   Tue, 17 Nov 2020 10:37:34 +0800
Message-ID: <CAC=cRTN77LAn-9-6rGukc2aUZQzx7oP9eKt_hJeb=wbnhGqObQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm/vmalloc: rework the drain logic
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Hillf Danton <hdanton@sina.com>,
        Michal Hocko <mhocko@suse.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
        Steven Rostedt <rostedt@goodmis.org>
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On Tue, Nov 17, 2020 at 6:00 AM Uladzislau Rezki (Sony)
<urezki@gmail.com> wrote:
>
> A current "lazy drain" model suffers from at least two issues.
>
> First one is related to the unsorted list of vmap areas, thus
> in order to identify the [min:max] range of areas to be drained,
> it requires a full list scan. What is a time consuming if the
> list is too long.
>
> Second one and as a next step is about merging all fragments
> with a free space. What is also a time consuming because it
> has to iterate over entire list which holds outstanding lazy
> areas.
>
> See below the "preemptirqsoff" tracer that illustrates a high
> latency. It is ~24 676us. Our workloads like audio and video
> are effected by such long latency:

This seems like a real problem.  But I found there's long latency
avoidance mechanism in the loop in __purge_vmap_area_lazy() as
follows,

        if (atomic_long_read(&vmap_lazy_nr) < resched_threshold)
            cond_resched_lock(&free_vmap_area_lock);

If it works properly, the latency problem can be solved.  Can you
check whether this doesn't work for you?

Best Reagrds,
Huang, Ying

<snip>