linux-kernel - Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for MPOL_PREFERRED_MANY by adding a NO

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Date:   Wed, 3 Mar 2021 20:07:17 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        Andi leen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for
 MPOL_PREFERRED_MANY by adding a NO_SLOWPATH gfp bit

Hi Michal,

On Wed, Mar 03, 2021 at 12:39:57PM +0100, Michal Hocko wrote:
> On Wed 03-03-21 18:20:58, Feng Tang wrote:
> > When doing broader test, we noticed allocation slowness in one test
> > case that malloc memory with size which is slightly bigger than free
> > memory of targeted nodes, but much less then the total free memory
> > of system.
> > 
> > The reason is the code enters the slowpath of __alloc_pages_nodemask(),
> > which takes quite some time. As alloc_pages_policy() will give it a 2nd
> > try with NULL nodemask, so there is no need to enter the slowpath for
> > the first try. Add a new gfp bit to skip the slowpath, so that user cases
> > like this can leverage.
> > 
> > With it, the malloc in such case is much accelerated as it never enters
> > the slowpath.
> > 
> > Adding a new gfp_mask bit is generally not liked, and another idea is to
> > add another nodemask to struct 'alloc_context', so it has 2: 'preferred-nmask'
> > and 'fallback-nmask', and they will be tried in turn if not NULL, with
> > it we can call __alloc_pages_nodemask() only once.
> 
> Yes, it is very much disliked. Is there any reason why you cannot use
> GFP_NOWAIT for that purpose?

I did try that at the first place, but it didn't obviously change the slowness.
I assumed the direct claim was still involved as GFP_NOWAIT only impact kswapd
reclaim.

Thanks,
Feng


> -- 
> Michal Hocko
> SUSE Labs