linux-kernel - Re: [PATCH v2 1/7] memcg: reduce memory size of mem_cgroup_events

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Message-ID: <lyxw3jphvncnfgenvpocl2srs2vtm4frmv7yyieel544apwaub@mrddepkseafr>
Date: Fri, 26 Apr 2024 18:15:14 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/7] memcg: reduce memory size of
 mem_cgroup_events_index

On Fri, Apr 26, 2024 at 05:42:48PM -0700, Yosry Ahmed wrote:
> On Fri, Apr 26, 2024 at 5:37 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > mem_cgroup_events_index is a translation table to get the right index of
> > the memcg relevant entry for the general vm_event_item. At the moment,
> > it is defined as integer array. However on a typical system the max
> > entry of vm_event_item (NR_VM_EVENT_ITEMS) is 113, so we don't need to
> > use int as storage type of the array. For now just use int8_t as type
> > and add a BUILD_BUG_ON() and will switch to short once NR_VM_EVENT_ITEMS
> > touches 127.
> 
> Any reason not to use uint8_t (or simply u8) and U8_MAX (instead of
> the hardcoded 127)?
> 

Just to keep the error check simple i.e. (index < 0). If we hit 127 then
we can switch to uint8_t and S8_MAX as error. Though 127 should be
replaced by S8_MAX. Somehow I was grep'ing for INT8*MAX variants.
Anyways if there is more support for uint8_t, I will change otherwise I
will keep as is.