linux-kernel - Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid long block

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Date:   Wed, 20 Dec 2017 09:35:39 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     Michal Hocko <mhocko@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid
 long block



On 12/18/17 12:41 AM, Michal Hocko wrote:
> On Sat 16-12-17 23:09:25, Kirill A. Shutemov wrote:
>> On Sat, Dec 16, 2017 at 12:45:25PM +0100, Michal Hocko wrote:
>>> On Sat 16-12-17 04:04:10, Yang Shi wrote:
> [...]
>>>> Shall we add "cond_resched()" in unmap_vmas(), i.e for every 100 vmas? It
>>>> may improve the responsiveness a little bit for non-preempt kernel, although
>>>> it still can't release the semaphore.
>>>
>>> We already do, once per pmd (see zap_pmd_range).
>>
>> It doesn't help. We would need to find a way to drop mmap_sem, if we're
>> holding it way too long. And doing it on per-vma count basis is not right
>> call. It won't address issue with single huge vma.
> 
> Absolutely agreed. I just wanted to point out that a new cond_resched is
> not really needed. One way to reduce the lock starvation is to use range
> locking.

It looks the range locking development is stalled?

Yang

> 
>> Do we have any instrumentation that would help detect starvation on a
>> rw_semaphore?
> 
> I am afraid we don't.
>