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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 13 May 2024 10:33:46 -0700
Message-ID: <CAHk-=wiZpyWvC-nh4CPhKkPLMwWb_W00NDMopuxVNTnGB7fYeg@mail.gmail.com>
Subject: Re: [GIT PULL] slab updates for 6.10
To: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, 
	Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>, 
	Andrew Morton <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, 
	LKML <linux-kernel@vger.kernel.org>, patches@lists.linux.dev, 
	Roman Gushchin <roman.gushchin@linux.dev>, Hyeonggon Yoo <42.hyeyoo@gmail.com>, 
	Chengming Zhou <chengming.zhou@linux.dev>
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On Thu, 9 May 2024 at 07:25, Vlastimil Babka <vbabka@suse.cz> wrote:
>
>   To avoid affecting fast paths with another shared counter (attempted in the
>   past) or complex partial list traversal schemes that allow rescheduling, the
>   chosen solution resorts to approximation - when the partial list is over
>   10000 slabs long, we will only traverse first 5000 slabs from head and tail
>   each and use the average of those to estimate the whole list. Both head and
>   tail are used as the slabs near head to tend to have more free objects than
>   the slabs towards the tail.

I suspect you could have cut this down by an order of magnitude, and
made the limit be just 1k slabs rather than 10k slabs. Or even
_another_ order of magnitude smaller.

Somebody was being a bit too worried about approximations, methinks -
but I think the real worry goes the other way, where it's practically
so hard to even hit the approximation situation that it gets no
testing at all.

IOW, I suspect it's better to be explicit about approximations, and
have people aware of it, rather than be overly cautious and have it be
a special case that almost never triggers in any normal loads.

But pulled.

              Linus