linux-kernel - Re: [PATCH 00/10] mm: PCP high auto-tuning

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Message-Id: <20230921084617.b1129a46de247e4a6f0098af@linux-foundation.org>
Date:   Thu, 21 Sep 2023 08:46:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Arjan Van De Ven <arjan@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Johannes Weiner <jweiner@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 00/10] mm: PCP high auto-tuning

On Thu, 21 Sep 2023 21:32:35 +0800 "Huang, Ying" <ying.huang@...el.com> wrote:

> >   : It's possible that PCP high auto-tuning doesn't work well for some
> >   : workloads.  So, when PCP high is tuned by hand via the sysctl knob,
> >   : the auto-tuning will be disabled.  The PCP high set by hand will be
> >   : used instead.
> >
> >   Is it a bit hacky to disable autotuning when the user alters
> >   pcp-high?  Would it be cleaner to have a separate on/off knob for
> >   autotuning?
> 
> This was suggested by Mel Gormon,
> 
> https://lore.kernel.org/linux-mm/20230714140710.5xbesq6xguhcbyvi@techsingularity.net/
> 
> "
> I'm not opposed to having an adaptive pcp->high in concept. I think it would
> be best to disable adaptive tuning if percpu_pagelist_high_fraction is set
> though. I expect that users of that tunable are rare and that if it *is*
> used that there is a very good reason for it.
> "
> 
> Do you think that this is reasonable?

I suppose so, if it's documented!

Documentation/admin-guide/sysctl/vm.rst describes
percpu_pagelist_high_fraction.