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Date: Tue, 29 Jun 2010 18:17:38 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Tejun Heo <tj@kernel.org>
Cc: torvalds@linux-foundation.org, mingo@elte.hu, linux-kernel@vger.kernel.org,
       jeff@garzik.org, akpm@linux-foundation.org, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com,
       oleg@redhat.com, axboe@kernel.dk, dwalker@codeaurora.org,
       stefanr@s5r6.in-berlin.de, florian@mickler.org, andi@firstfloor.org,
       mst@redhat.com, randy.dunlap@oracle.com
Subject: Re: [PATCH 12/35] workqueue: update cwq alignement
Message-ID: <20100629161735.GM5318@nowhere>
References: <1277759063-24607-1-git-send-email-tj@kernel.org> <1277759063-24607-13-git-send-email-tj@kernel.org> <20100628224755.GA10104@nowhere> <4C29A33D.2060407@kernel.org> <20100629123632.GB5318@nowhere> <4C2A145E.8030608@kernel.org> <20100629154738.GJ5318@nowhere> <4C2A1672.20700@kernel.org> <20100629160100.GL5318@nowhere> <4C2A1ABF.8050504@kernel.org>
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On Tue, Jun 29, 2010 at 06:09:35PM +0200, Tejun Heo wrote:
> Hello,
> 
> On 06/29/2010 06:01 PM, Frederic Weisbecker wrote:
> > So, imagine you allocate your struct with alloc_percpu(align).
> > 
> > The per cpu pointer is 0x400 (purely imagination).
> > 
> > Now you have two cpus and they have the following base offsets for
> > per cpu allocations:
> > 
> > CPU 0 = 0xf1000000
> > CPU 1 = 0xf2000000
> > 
> > So, the true pointers for your cpu workqueue structs will be:
> > 
> > CPU 0 = 0xf1000400
> > CPU 1 = 0xf2000400
> > 
> > These addresses are aligned like you wanted to, and it seems it is what
> > matters, to store these addresses in the work flags.
> 
> Yes.
> 
> > So why does the size of the struct need to be aligned too?
> 
> Where am I doing that?


Ah well, yesterday there was a BUILD_BUG_ON on init_workqueues that checked
this structure size was well aligned. Now that I check again, it seems to have
disappeared after you updated the patch.


> 
> > All you want is that the two above addresses are aligned. Now why
> > the size of the struct itself needs this alignment too. That's the
> > obscure point for me. If it's useless, this could avoid all this
> > alignment maintainance, except during the allocation itself.
> 
> What alignment maintenance?  Are you talking about the UP code?  If
> you're talking about the UP code, the ugliness there is because the
> current UP __alloc_percpu() can't honor the alignment parameter.


Heh no, it's about a leftover in your patchset that you have fixed
now.


> Heh, it seems I'm still lost.  Care to give one more shot at it?  :-)


My bad, I haven't looked your updated patch in detail... :)

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