linux-kernel - Re: [PATCH v2] mm/hugetlb: defer free_huge

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Date:   Mon, 16 Dec 2019 14:13:40 -0500
From:   Waiman Long <longman@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aneesh.kumar@...ux.ibm.com
Subject: Re: [PATCH v2] mm/hugetlb: defer free_huge_page() to a workqueue

On 12/16/19 2:08 PM, Mike Kravetz wrote:
> On 12/16/19 8:17 AM, Davidlohr Bueso wrote:
>> On Mon, 16 Dec 2019, Michal Hocko wrote:
>>> I am afraid that work_struct is too large to be stuffed into the struct
>>> page array (because of the lockdep part).
>> Yeah, this needs to be done without touching struct page.
>>
>> Which is why I had done the stack allocated way in this patch, but we
>> cannot wait for it to complete in irq, so that's out the window. Andi
>> had suggested percpu allocated work items, but having played with the
>> idea over the weekend, I don't see how we can prevent another page being
>> freed on the same cpu before previous work on the same cpu is complete
>> (cpu0 wants to free pageA, schedules the work, in the mean time cpu0
>> wants to free pageB and workerfn for pageA still hasn't been called).
>>
>>> I think that it would be just safer to make hugetlb_lock irq safe. Are
>>> there any other locks that would require the same?
>> It would be simpler. Any performance issues that arise would probably
>> be only seen in microbenchmarks, assuming we want to have full irq safety.
>> If we don't need to worry about hardirq, then even better.
>>
>> The subpool lock would also need to be irq safe.
> I do think we need to worry about hardirq.  There are no restruictions that
> put_page can not be called from hardirq context. 
>
> I am concerned about the latency of making hugetlb_lock (and potentially
> subpool lock) hardirq safe.  When these locks were introduced (before my
> time) the concept of making them irq safe was not considered.  Recently,
> I learned that the hugetlb_lock is held for a linear scan of ALL hugetlb
> pages during a cgroup reparentling operation.  That is just too long.
>
> If there is no viable work queue solution, then I think we would like to
> restructure the hugetlb locking before a change to just make hugetlb_lock
> irq safe.  The idea would be to split the scope of what is done under
> hugetlb_lock.  Most of it would never be executed in irq context.  Then
> have a small/limited set of functionality that really needs to be irq
> safe protected by an irq safe lock.
>
Please take a look at my recently posted patch to see if that is an
acceptable workqueue based solution.

Thanks,
Longman