linux-kernel - Re: [PATCH] mm, percpu: do not consider sleepable allocations atomic

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Message-ID: <Z69kkwVJuRIWjJ8K@tiehlicka>
Date: Fri, 14 Feb 2025 16:43:15 +0100
From: Michal Hocko <mhocko@...e.com>
To: Tejun Heo <tj@...nel.org>
Cc: Dennis Zhou <dennis@...nel.org>, Filipe Manana <fdmanana@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, percpu: do not consider sleepable allocations atomic

On Wed 12-02-25 11:30:08, Tejun Heo wrote:
> Hello,
> 
> On Wed, Feb 12, 2025 at 09:53:20PM +0100, Michal Hocko wrote:
> ...
> > > Hmm... you'd a better judge on whether that'd be okay or not but it does
> > > bother me that we might be increasing the chance of allocation failures for
> > > GFP_KERNEL users at least under memory pressure.
> > 
> > Nope, this will not change the allocation failure mode. Reclaim
> > constrains do not change the failure mode they just change how much the
> > allocation might struggle to reclaim to succeed. 
> >
> > My undocumented assumption (another dept on my end) is that pcp
> > allocations are no hot paths. So the worst case is that GFP_KERNEL
> > pcp_allocation could have been satisfied _easier_ (i.e. faster) because
> > it could have reclaimed fs/io caches and now it needs to rely on kswapd
> > to do that on memory tight situations. On the other hand we have a
> > situation when NOIO/FS allocations fail prematurely so there is
> > certainly some pros and cons.
> 
> I'm having a hard time following. Are you saying that it won't increase the
> likelihood of allocation failures even under memory pressure but that it
> might just make allocations take longer to succeed?

yes, this is like any other NOFS/NOIO allocation non-costly
(<=PAGE_ALLOC_COSTLY_ORDER) which effectively never fail.

-- 
Michal Hocko
SUSE Labs