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

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Date:	Tue, 27 Oct 2015 20:07:38 +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, Oct 23, 2015 at 01:11:45PM +0200, Michal Hocko wrote:
> > > > The problem here is not lack
> > > > of execution resource but concurrency management misunderstanding the
> > > > situation. 
> > > 
> > > And this sounds like a bug to me.
> > 
> > I don't know.  I can be argued either way, the other direction being a
> > kernel thread going RUNNING non-stop is buggy.  Given how this has
> > been a complete non-issue for all the years, I'm not sure how useful
> > plugging this is.
> 
> Well, I guess we haven't noticed because this is a pathological case. It
> also triggers OOM livelocks which were not reported in the past either.
> You do not reach this state normally unless you rely _want_ to kill your
> machine

I don't think we can say this is a pathological case. Customers' serves
might have hit this state. We have no code for warning this state.

> 
> And vmstat is not the only instance. E.g. sysrq oom trigger is known
> to stay behind in similar cases. It should be changed to a dedicated
> WQ_MEM_RECLAIM wq and it would require runnable item guarantee as well.
> 

Well, this seems to be the cause of SysRq-f being unresponsive...
http://lkml.kernel.org/r/201411231349.CAG78628.VFQFOtOSFJMOLH@I-love.SAKURA.ne.jp

Picking up from http://lkml.kernel.org/r/201506112212.JAG26531.FLSVFMOQJOtOHF@I-love.SAKURA.ne.jp
----------
[  515.536393] Showing busy workqueues and worker pools:
[  515.538185] workqueue events: flags=0x0
[  515.539758]   pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=8/256
[  515.541872]     pending: vmpressure_work_fn, console_callback, vmstat_update, flush_to_ldisc, push_to_pool, moom_callback, sysrq_reinject_alt_sysrq, fb_deferred_io_work
[  515.546684] workqueue events_power_efficient: flags=0x80
[  515.548589]   pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=2/256
[  515.550829]     pending: neigh_periodic_work, check_lifetime
[  515.552884] workqueue events_freezable_power_: flags=0x84
[  515.554742]   pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=1/256
[  515.556846]     in-flight: 3837:disk_events_workfn
[  515.558665] workqueue writeback: flags=0x4e
[  515.560291]   pwq 16: cpus=0-7 flags=0x4 nice=0 active=2/256
[  515.562271]     in-flight: 3812:bdi_writeback_workfn bdi_writeback_workfn
[  515.564544] workqueue xfs-data/sda1: flags=0xc
[  515.566265]   pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=4/256
[  515.568359]     in-flight: 374(RESCUER):xfs_end_io, 3759:xfs_end_io, 26:xfs_end_io, 3836:xfs_end_io
[  515.571018]   pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=1/256
[  515.573113]     in-flight: 179:xfs_end_io
[  515.574782] pool 2: cpus=1 node=0 flags=0x0 nice=0 workers=4 idle: 3790 237 3820
[  515.577230] pool 6: cpus=3 node=0 flags=0x0 nice=0 workers=5 manager: 219
[  515.579488] pool 16: cpus=0-7 flags=0x4 nice=0 workers=3 idle: 356 357
----------
We want immediate execution guarantee for not only vmstat_update and
moom_callback but also vmstat_shepherd and console_callback?

> > > 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
> > 
> > Seriously, nobody goes full-on RUNNING.
> 
> Looping with cond_resched seems like general pattern in the kernel when
> there is no clear source to wait for. We have io_schedule when we know
> we should wait for IO (in case of congestion) but this is not necessarily
> the case - as you can see here. What should we wait for? A short nap
> without actually waiting on anything sounds like a dirty workaround to
> me.

Can't we have a waitqueue like
http://lkml.kernel.org/r/201510142121.IDE86954.SOVOFFQOFMJHtL@I-love.SAKURA.ne.jp ?
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