linux-kernel - Re: [PATCH 08/10] percpu: change the number of pages marked in the first

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Message-ID: <20170724201341.GC91613@dennisz-mbp.dhcp.thefacebook.com>
Date:   Mon, 24 Jul 2017 16:13:42 -0400
From:   Dennis Zhou <dennisz@...com>
To:     Tejun Heo <tj@...nel.org>
CC:     Christoph Lameter <cl@...ux.com>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Dennis Zhou <dennisszhou@...il.com>
Subject: Re: [PATCH 08/10] percpu: change the number of pages marked in the
 first_chunk bitmaps

On Mon, Jul 17, 2017 at 03:26:02PM -0400, Tejun Heo wrote:
> > This patch changes the allocator to only mark allocated pages for the
> > region the population bitmap is used for. Prior, the bitmap was marked
> > completely used as the first chunk was allocated and immutable. This is
> > misleading because the first chunk may not be completely filled.
> > Additionally, with moving the base_addr up in the previous patch, the
> > population map no longer corresponds to what is being checked.
> 
> This in isolation makes sense although the rationale isn't clear from
> the description.  Is it a mere cleanup or is this needed to enable
> further changes?

This change is clean up to make sure there is no misunderstanding
between what part of the bitmap actually is meaningful and the actual
size of the bitmap.


> > pcpu_nr_empty_pop_pages is used to ensure there are a handful of free
> > pages around to serve atomic allocations. A new field, nr_empty_pop_pages,
> > is added to the pcpu_chunk struct to keep track of the number of empty
> > pages. This field is needed as the number of empty populated pages is
> > globally kept track of and deltas are used to update it. This new field
> > is exposed in percpu_stats.
> 
> But I can't see why this is being added or why this is in the same
> patch with the previous change.
> 

I've split this out into another patch.

> > Now that chunk->nr_pages is the number of pages the chunk is serving, it
> > is nice to use this in the work function for population and freeing of
> > chunks rather than use the global variable pcpu_unit_pages.
> 
> The same goes for the above part.  It's fine to collect misc changes
> into a patch when they're trivial and related in some ways but the
> content of this patch seems a bit random.

This change is needed in the same patch because chunk->nr_populated no
longer is set to pcpu_unit_pages. The checks would check the dynamic
chunk and then try to populate. Those checks should be checking against
the size of the region being served which is nr_pages.

Thanks,
Dennis