linux-kernel - Re: [RFT PATCH] blkio: alloc per cpu data from worker thread context( Re: kvm deadlock)

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Message-ID: <20111220145024.GA16946@redhat.com>
Date:	Tue, 20 Dec 2011 09:50:24 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Nate Custer <nate@...nel.net>, Jens Axboe <axboe@...nel.dk>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH] blkio: alloc per cpu data from worker thread
 context( Re: kvm deadlock)

On Mon, Dec 19, 2011 at 02:56:35PM -0800, Tejun Heo wrote:
> Hello, Vivek.
> 
> On Mon, Dec 19, 2011 at 01:27:17PM -0500, Vivek Goyal wrote:
> > Ok, that's good to know. If per cpu allocator can support this use case,
> > it will be good for 3.3 onwards. This seems to be right way to go to fix
> > the problem.
> 
> Ummm... if we're gonna make percpu usable w/ GFP_NOIO, the right
> interim solution would be making a simplistic mempool so that later
> when percpu can do it, it can be swapped easily.  I really can't see
> much benefit of adding refcnting on top of everything just for this.

Ok. So are you suggesting that I should write a simple mempool kind of
implementation of my own for group and per cpu data allocation. Keep
certain number of elements in the cache and trigger a worker thread to
allocate more elements once minimum number of elements go below
threshold. If pool has run out of pre-allocated elements then allocation
will fail and IO will be accounted to root group?

I am looking at the mempool implementation (mempool_create()) and looks
like that is not suitable for my use case. mempool_alloc() will call into
alloc function provided by me and pass the flags. I can't implement an
alloc function and honor that flag as per cpu alloc does not take any
flags.

So IIUC, existing mempool implementation is not directly usable for my
requirement and I need to write some code of my own for the caching
layer which always allocates objects from reserve and fills in the
pool asynchronously with the help of a worker thread.

Thanks
Vivek
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