linux-kernel - Re: [PATCH 1/2] mm/slub: wake up kswapd for initial high order allocation

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Message-ID: <f1423efc-3c60-c03e-0d81-f2e8fcccbcd6@suse.cz>
Date:   Mon, 28 Aug 2017 12:04:41 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 1/2] mm/slub: wake up kswapd for initial high order
 allocation

On 08/28/2017 03:11 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> slub uses higher order allocation than it actually needs. In this case,
> we don't want to do direct reclaim to make such a high order page since
> it causes a big latency to the user. Instead, we would like to fallback
> lower order allocation that it actually needs.
> 
> However, we also want to get this higher order page in the next time
> in order to get the best performance and it would be a role of
> the background thread like as kswapd and kcompactd. To wake up them,
> we should not clear __GFP_KSWAPD_RECLAIM.
> 
> Unlike this intention, current code clears __GFP_KSWAPD_RECLAIM so fix it.
> 
> Note that this patch does some clean up, too.
> __GFP_NOFAIL is cleared twice so remove one.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Hm, so this seems to revert Mel's 444eb2a449ef ("mm: thp: set THP defrag
by default to madvise and add a stall-free defrag option") wrt the slub
allocate_slab() part. AFAICS the intention in Mel's patch was that he
removed a special case in __alloc_page_slowpath() where including
__GFP_THISNODE and lacking ~__GFP_DIRECT_RECLAIM effectively means also
lacking __GFP_KSWAPD_RECLAIM. The commit log claims that slab/slub might
change behavior so he moved the removal of __GFP_KSWAPD_RECLAIM to them.

But AFAICS, only slab uses __GFP_THISNODE, while slub doesn't. So your
patch would indeed revert an unintentional change of Mel's commit. Is it
right or do I miss something?

> ---
>  mm/slub.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 0dc7397..e1e442c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1578,8 +1578,12 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	 * so we fall-back to the minimum order allocation.
>  	 */
>  	alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL;
> -	if ((alloc_gfp & __GFP_DIRECT_RECLAIM) && oo_order(oo) > oo_order(s->min))
> -		alloc_gfp = (alloc_gfp | __GFP_NOMEMALLOC) & ~(__GFP_RECLAIM|__GFP_NOFAIL);
> +	if (oo_order(oo) > oo_order(s->min)) {
> +		if (alloc_gfp & __GFP_DIRECT_RECLAIM) {
> +			alloc_gfp |= __GFP_NOMEMALLOC;
> +			alloc_gfp &= ~__GFP_DIRECT_RECLAIM;
> +		}
> +	}
>  
>  	page = alloc_slab_page(s, alloc_gfp, node, oo);
>  	if (unlikely(!page)) {
>