linux-kernel - Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation and remove stats

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Message-Id: <20120307150806.1804a8b1.akpm@linux-foundation.org>
Date:	Wed, 7 Mar 2012 15:08:06 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, axboe@...nel.dk, hughd@...gle.com,
	avi@...hat.com, nate@...nel.net, cl@...ux-foundation.org,
	linux-kernel@...r.kernel.org, dpshah@...gle.com,
	ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation
 and remove stats_lock

On Wed, 7 Mar 2012 14:56:29 -0800
Tejun Heo <tj@...nel.org> wrote:

> On Wed, Mar 07, 2012 at 02:42:29PM -0500, Vivek Goyal wrote:
> > blk-cgroup: Alloc per cpu stats from worker thread in a delayed manner
> > 
> > Current per cpu stat allocation assumes GFP_KERNEL allocation flag. But in
> > IO path there are times when we want GFP_NOIO semantics. As there is no
> > way to pass the allocation flags to alloc_percpu(), this patch delays the
> > allocation of stats using a worker thread.
> > 
> > v2-> tejun suggested following changes. Changed the patch accordingly.
> > 	- move alloc_node location in structure
> > 	- reduce the size of names of some of the fields
> > 	- Reduce the scope of locking of alloc_list_lock
> > 	- Simplified stat_alloc_fn() by allocating stats for all
> > 	  policies in one go and then assigning these to a group.
> > 
> > v3 -> Andrew suggested to put some comments in the code. Also raised
> >       concerns about trying to allocate infinitely in case of allocation
> >       failure. I have changed the logic to sleep for 10ms before retrying.
> >       That should take care of non-preemptible UP kernels.
> > 
> > v4 -> Tejun had more suggestions.
> > 	- drop list_for_each_entry_all()
> > 	- instead of msleep() use queue_delayed_work()
> > 	- Some cleanups realted to more compact coding.
> >  
> > v5-> tejun suggested more cleanups leading to more compact code.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> 
> Looks good to me.  I'll roll it into the stats series.  Andrew, if you
> still have objections, please scream.

Just the "msleep adds to load average" issue needs thinking about.

It's rather a miserable patch :(
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