linux-kernel - Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

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Date:   Mon, 26 Aug 2019 09:43:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, hannes@...xchg.org,
        vbabka@...e.cz, rientjes@...gle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

On Thu 22-08-19 08:33:40, Yang Shi wrote:
> 
> 
> On 8/22/19 1:04 AM, Michal Hocko wrote:
> > On Thu 22-08-19 01:55:25, Yang Shi wrote:
[...]
> > > And, they seems very common with the common workloads when THP is
> > > enabled.  A simple run with MariaDB test of mmtest with THP enabled as
> > > always shows it could generate over fifteen thousand deferred split THPs
> > > (accumulated around 30G in one hour run, 75% of 40G memory for my VM).
> > > It looks worth accounting in MemAvailable.
> > OK, this makes sense. But your above numbers are really worrying.
> > Accumulating such a large amount of pages that are likely not going to
> > be used is really bad. They are essentially blocking any higher order
> > allocations and also push the system towards more memory pressure.
> 
> That is accumulated number, during the running of the test, some of them
> were freed by shrinker already. IOW, it should not reach that much at any
> given time.

Then the above description is highly misleading. What is the actual
number of lingering THPs that wait for the memory pressure in the peak?
 
> > IIUC deferred splitting is mostly a workaround for nasty locking issues
> > during splitting, right? This is not really an optimization to cache
> > THPs for reuse or something like that. What is the reason this is not
> > done from a worker context? At least THPs which would be freed
> > completely sound like a good candidate for kworker tear down, no?
> 
> Yes, deferred split THP was introduced to avoid locking issues according to
> the document. Memcg awareness would help to trigger the shrinker more often.
> 
> I think it could be done in a worker context, but when to trigger to worker
> is a subtle problem.

Why? What is the problem to trigger it after unmap of a batch worth of
THPs?
-- 
Michal Hocko
SUSE Labs