linux-kernel - Re: [PATCH] mm,vmscan: Use accurate values for zone

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Date:	Fri, 23 Oct 2015 21:25:11 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, htejun@...il.com
Cc:	cl@...ux.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, rientjes@...gle.com,
	oleg@...hat.com, kwalker@...hat.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, vdavydov@...allels.com, skozina@...hat.com,
	mgorman@...e.de, riel@...hat.com
Subject: Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks

Michal Hocko wrote:
> On Fri 23-10-15 19:36:30, Tejun Heo wrote:
> > Hello, Michal.
> > 
> > On Fri, Oct 23, 2015 at 10:33:16AM +0200, Michal Hocko wrote:
> > > Ohh, OK I can see wq_worker_sleeping now. I've missed your point in
> > > other email, sorry about that. But now I am wondering whether this
> > > is an intended behavior. The documentation says:
> > 
> > This is.
> > 
> > >   WQ_MEM_RECLAIM
> > > 
> > >         All wq which might be used in the memory reclaim paths _MUST_
> > >         have this flag set.  The wq is guaranteed to have at least one
> > >         execution context regardless of memory pressure.
> > > 
> > > Which doesn't seem to be true currently, right? Now I can see your patch
> > 
> > It is true.
> > 
> > > to introduce WQ_IMMEDIATE but I am wondering which WQ_MEM_RECLAIM users
> > > could do without WQ_IMMEDIATE? I mean all current workers might be
> > > looping in the page allocator and it seems possible that WQ_MEM_RECLAIM
> > > work items might be waiting behind them so they cannot help to relieve
> > > the memory pressure. This doesn't sound right to me. Or I am completely
> > > confused and still fail to understand what is WQ_MEM_RECLAIM supposed to
> > > be used for.
> > 
> > It guarantees that there always is enough execution resource to
> > execute a work item from that workqueue. 
> 
> OK, strictly speaking the rescuer is there but it is kind of pointless
> if it doesn't fire up and do a work.
> 
> > The problem here is not lack
> > of execution resource but concurrency management misunderstanding the
> > situation. 
> 
> And this sounds like a bug to me.
> 
> > This also can be fixed by teaching concurrency management
> > to be a bit smarter - e.g. if a work item is burning a lot of CPU
> > cycles continuously or pool hasn't finished a work item over a certain
> > amount of time, automatically ignore the in-flight work item for the
> > purpose of concurrency management; however, this sort of inter-work
> > item busy waits are so extremely rare and undesirable that I'm not
> > sure the added complexity would be worthwhile.
> 
> Don't we have some IO related paths which would suffer from the same
> problem. I haven't checked all the WQ_MEM_RECLAIM users but from the
> name I would expect they _do_ participate in the reclaim and so they
> should be able to make a progress. Now if your new IMMEDIATE flag will
> guarantee that then I would argue that it should be implicit for
> WQ_MEM_RECLAIM otherwise we always risk a similar situation. What would
> be a counter argument for doing that?

WQ_MEM_RECLAIM only guarantees that a "struct task_struct" is preallocated
in order to avoid failing to allocate it on demand due to a GFP_KERNEL
allocation? Is this correct?

WQ_CPU_INTENSIVE only guarantees that work items don't participate in
concurrency management in order to avoid failing to wake up a "struct
task_struct" which will process the work items? Is this correct?

Is Michal's question "does it make sense to use WQ_MEM_RECLAIM without
WQ_CPU_INTENSIVE"? In other words, any "struct task_struct" which calls
rescuer_thread() must imply WQ_CPU_INTENSIVE in order to avoid failing to
wake up due to being participated in concurrency management?
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