linux-kernel - Re: [PATCH v2] mm: Add configuration to control whether vmpressure notifier is enabled

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Message-ID: <YSzh31BasoxUQXAu@dhcp22.suse.cz>
Date:   Mon, 30 Aug 2021 15:49:19 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     yong w <yongw.pur@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>, alexs@...nel.org,
        Wei Yang <richard.weiyang@...il.com>, Hui Su <sh_def@....com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        wang.yong12@....com.cn, Cgroups <cgroups@...r.kernel.org>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, yang.yang29@....com.cn
Subject: Re: [PATCH v2] mm: Add configuration to control whether vmpressure
 notifier is enabled

On Sun 22-08-21 17:46:08, yong w wrote:
> > All those reasons should be a part of the changelog.
> >....
> > I am not sure these are sufficient justifications but that is something
> > to discuss. And hence it should be a part of the changelog.
> >
> OK, These reasons will be added to the patch notesin later versions.
> 
> > > 3. In the case where the user does not need vmpressure,  vmpressure
> > > calculation is additional overhead.
> >
> > You should quantify that and argue why that overhead cannot be further
> > reduced without config/boot time knobs.
> >
> The test results of the previously used PFT tool may not be obvious.
> Is there a better way to quantify it?

This is a question for you to answer I am afraid. You want to add a
configuration option and (as explained) that is not free of cost from
the maintenance POV. There must a very good reason to do that.

> > > In some special scenes with tight memory, vmpressure will be executed
> > > frequently.we use "likely" and "inline"
> > > to improve the performance of the kernel, why not reduce some
> > > unnecessary calculations?
> >
> > I am all for improving the code. Is it possible to do it by other means?
> > E.g. reduce a potential overhead when there no events registered?
> Yes, the method you mentioned may be feasible, but it does not conflict
> with this patch.

It is not in conflict but runtime overhead reduction without more burden
on the configurability is usually a preferred approach.
-- 
Michal Hocko
SUSE Labs