linux-kernel - Re: [RFC][PATCH v3 2/5] mm/zsmalloc: remove shrinker compaction callbacks

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Message-ID: <20160315021957.GA25154@bbox>
Date:	Tue, 15 Mar 2016 11:19:57 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v3 2/5] mm/zsmalloc: remove shrinker compaction
 callbacks

On Tue, Mar 15, 2016 at 10:05:42AM +0900, Sergey Senozhatsky wrote:
> On (03/15/16 09:52), Minchan Kim wrote:
> [..]
> > > > I suggested to remove shrinker compaction but while I review your
> > > > first patch in this thread, I thought we need upper-bound to
> > > > compact zspage so background work can bail out for latency easily.
> > > > IOW, the work could give up the job. In such case, we might need
> > > > fall-back scheme to continue the job. And I think that could be
> > > > a shrinker.
> > > > 
> > > > What do you think?
> > > 
> > > wouldn't this unnecessarily complicate the whole thing? we would
> > > have
> > >  a) a compaction that can be triggered by used space
> > 
> > Maybe, user space? :)
> 
> haha, yes!  sorry, I do quite a lot of typos.
> 
> > >  b) a compaction from zs_free() that can bail out
> > >  c) a compaction triggered by the shrinker.
> > > 
> > > all 3 three can run simultaneously.
> > 
> > Yeb.
> > 
> > > 
> > > 
> > > _if_ we can keep every class below its watermark, we can reduce the
> > > need of "c)".
> > 
> > But the problem is timing. We cannot guarantee when background
> > compaction triggers while shrinker is interop with VM so we should
> > do the job instantly for the system.
> 
> we can have pool's compaction-kthread that we will wake_up()
> every time we need a compaction, with no dependency on workqueue
> or shrinker.

Hmm, I don't think it can work either because wake_up doesn't
guarantee instant execution of the thread.
I think it would be better to have both direct/background
compaction.