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

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Date:   Mon, 18 Dec 2017 09:41:19 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Yang Shi <yang.s@...baba-inc.com>, 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 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.

> Do we have any instrumentation that would help detect starvation on a
> rw_semaphore?

I am afraid we don't.

-- 
Michal Hocko
SUSE Labs