linux-kernel - Re: [PATCH 07/11] mm/page_alloc: Remove duplicate checks if migratetype should be isolated

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Date:   Thu, 15 Apr 2021 10:33:37 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 07/11] mm/page_alloc: Remove duplicate checks if
 migratetype should be isolated

On Wed, Apr 14, 2021 at 07:21:42PM +0200, Vlastimil Babka wrote:
> On 4/14/21 3:39 PM, Mel Gorman wrote:
> > Both free_pcppages_bulk() and free_one_page() have very similar
> > checks about whether a page's migratetype has changed under the
> > zone lock. Use a common helper.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> 
> Seems like for free_pcppages_bulk() this patch makes it check for each page on
> the pcplist
> - zone->nr_isolate_pageblock != 0 instead of local bool (the performance might
> be the same I guess on modern cpu though)
> - is_migrate_isolate(migratetype) for a migratetype obtained by
> get_pcppage_migratetype() which cannot be migrate_isolate so the check is useless.
> 
> As such it doesn't seem a worthwhile cleanup to me considering all the other
> microoptimisations?
> 

The patch was a preparation patch for the rest of the series to avoid code
duplication and to consolidate checks together in one place to determine
if they are even correct.

Until zone_pcp_disable() came along, it was possible to have isolated PCP
pages in the lists even though zone->nr_isolate_pageblock could be 0 during
memory hot-remove so the split in free_pcppages_bulk was not necessarily
correct at all times.

The remaining problem is alloc_contig_pages, it does not disable
PCPs so both checks are necessary. If that also disabled PCPs
then check_migratetype_isolated could be deleted but the cost to
alloc_contig_pages might be too high.

I'll delete this patch for now because it's relatively minor and there
should be other ways of keeping the code duplication down.

-- 
Mel Gorman
SUSE Labs