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 21:18:32 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.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>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Widawsky, Ben" <ben.widawsky@...el.com>,
        Andi leen <ak@...ux.intel.com>,
        "Williams, Dan J" <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

On Wed, Mar 03, 2021 at 01:32:11PM +0100, Michal Hocko wrote:
> On Wed 03-03-21 20:18:33, Feng Tang wrote:
> > On Wed, Mar 03, 2021 at 08:07:17PM +0800, Tang, Feng wrote:
> > > 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.
> 
> I assume you haven't really created gfp mask correctly. What was the
> exact gfp mask you have used?

The testcase is a malloc with multi-preferred-node policy, IIRC, the gfp
mask is HIGHUSER_MOVABLE originally, and code here ORs (__GFP_RETRY_MAYFAIL | __GFP_NOWARN).

As GFP_WAIT == __GFP_KSWAPD_RECLAIM, in this test case, the bit is already set.

> > 
> > One thing I tried which can fix the slowness is:
> > 
> > +	gfp_mask &= ~(__GFP_DIRECT_RECLAIM | __GFP_KSWAPD_RECLAIM);
> > 
> > which explicitly clears the 2 kinds of reclaim. And I thought it's too
> > hacky and didn't mention it in the commit log.
> 
> Clearing __GFP_DIRECT_RECLAIM would be the right way to achieve
> GFP_NOWAIT semantic. Why would you want to exclude kswapd as well? 

When I tried gfp_mask &= ~__GFP_DIRECT_RECLAIM, the slowness couldn't
be fixed.

Thanks,
Feng

> -- 
> Michal Hocko
> SUSE Labs