linux-kernel - Re: [dm-devel] [RFC PATCH 2/2] mm, mempool: do not throttle PF_LESS

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Message-ID: <20160728071711.GB31860@dhcp22.suse.cz>
Date:	Thu, 28 Jul 2016 09:17:12 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	NeilBrown <neilb@...e.com>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	dm-devel@...hat.com, Mikulas Patocka <mpatocka@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Ondrej Kozina <okozina@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [dm-devel] [RFC PATCH 2/2] mm, mempool: do not throttle
 PF_LESS_THROTTLE tasks

On Thu 28-07-16 07:33:19, NeilBrown wrote:
> On Thu, Jul 28 2016, Michal Hocko wrote:
> 
> > On Wed 27-07-16 13:43:35, NeilBrown wrote:
> >> On Mon, Jul 25 2016, Michal Hocko wrote:
> >> 
> >> > On Sat 23-07-16 10:12:24, NeilBrown wrote:
> > [...]
> >> So should there be a limit on dirty
> >> pages in the swap cache just like there is for dirty pages in any
> >> filesystem (the max_dirty_ratio thing) ??
> >> Maybe there is?
> >
> > There is no limit AFAIK. We are relying that the reclaim is throttled
> > when necessary.
> 
> Is that a bit indirect?

Yes it is. Dunno, how much of a problem is that, though.

> It is hard to tell without a clear big-picture.
> Something to keep in mind anyway.
> 
> >
> >> I think we'd end up with cleaner code if we removed the cute-hacks.  And
> >> we'd be able to use 6 more GFP flags!!  (though I do wonder if we really
> >> need all those 26).
> >
> > Well, maybe we are able to remove those hacks, I wouldn't definitely
> > be opposed.  But right now I am not even convinced that the mempool
> > specific gfp flags is the right way to go.
> 
> I'm not suggesting a mempool-specific gfp flag.  I'm suggesting a
> transient-allocation gfp flag, which would be quite useful for mempool.
> 
> Can you give more details on why using a gfp flag isn't your first choice
> for guiding what happens when the system is trying to get a free page
> :-?

If we get rid of throttle_vm_writeout then I guess it might turn out to
be unnecessary. There are other places which will still throttle but I
believe those should be kept regardless of who is doing the allocation
because they are helping the LRU scanning sane. I might be wrong here
and bailing out from the reclaim rather than waiting would turn out
better for some users but I would like to see whether the first approach
works reasonably well.
-- 
Michal Hocko
SUSE Labs