linux-kernel - Re: [PATCH] sched,numa,mm: revert to checking pmd/pte

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Date:   Sun, 11 Sep 2016 17:24:02 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Rik van Riel <riel@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org, peterz@...radead.org,
        mingo@...nel.org, aarcange@...hat.com
Subject: Re: [PATCH] sched,numa,mm: revert to checking pmd/pte_write instead
 of VMA flags

On Thu, Sep 08, 2016 at 09:30:53PM -0400, Rik van Riel wrote:
> Commit 4d9424669946 ("mm: convert p[te|md]_mknonnuma and remaining
> page table manipulations") changed NUMA balancing from _PAGE_NUMA
> to using PROT_NONE, and was quickly found to introduce a regression
> with NUMA grouping.
> 
> It was followed up by these changesets:
> 
> 53da3bc2ba9e ("mm: fix up numa read-only thread grouping logic")
> bea66fbd11af ("mm: numa: group related processes based on VMA flags instead of page table flags")
> b191f9b106ea ("mm: numa: preserve PTE write permissions across a NUMA hinting fault")
> 
> The first of those two changesets try alternate approaches to NUMA
> grouping, which apparently do not work as well as looking at the PTE
> write permissions.
> 
> The latter patch preserves the PTE write permissions across a NUMA
> protection fault. However, it forgets to revert the condition for
> whether or not to group tasks together back to what it was before
> 3.19, even though the information is now preserved in the page tables
> once again.
> 
> This patch brings the NUMA grouping heuristic back to what it was
> before changeset 4d9424669946, which the changelogs of subsequent
> changesets suggest worked best.
> 
> We have all the information again. We should probably use it.
> 

Patch looks ok other than the comment above the second hunk being out of
date. Out of curiousity, what workload benefitted from this? I saw a mix
of marginal results when I ran this on a 2-socket and 4-socket box.

-- 
Mel Gorman
SUSE Labs