Date: Tue, 1 Sep 2026 16:59:46 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@suse.com>
Cc: David Stevens <stevensd@google.com>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>, cgroups@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] memcg: Don't call schedule_work when no spinning is
 allowed
Message-ID: <20260901205946.GL3004@cmpxchg.org>
References: <20260831234339.280376-1-stevensd@google.com>
 <apYXCL_GJs9Ovu0v@linux.dev>
 <CAOiLmNEV8b3TW_Q7+h5mYrZ1Nw73XcPFPUsapeYObLYajq_PJw@mail.gmail.com>
 <20260901142552.GG3004@cmpxchg.org>
 <apbyZgB_Zu-DK8pN@tiehlicka>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <apbyZgB_Zu-DK8pN@tiehlicka>

On Tue, Sep 01, 2026 at 05:42:30PM +0200, Michal Hocko wrote:
> On Tue 01-09-26 10:25:52, Johannes Weiner wrote:
> > On Mon, Aug 31, 2026 at 06:04:57PM -0700, David Stevens wrote:
> [...]
> > > There would be no guarantee that memcg reclaim would ever be
> > > triggered, which also would also stop MEMCG_HIGH events from being
> > > generated. Overall that seems a more serious than just dropping
> > > userspace notifications like is done for MEMCG_MAX.
> > 
> > I'm leaning that way too. It's an indefinite error, and it's a freely
> > programmable surface.
> 
> I am really curious about the indefinite error side of things. It has
> been my understanding that these NMI safe charges are a) rare and b)
> there is userspace running so eventually any discrepancies would
> resolve so the excess is temporary.

So I think the question is what limits the error in both space and
time. When you say it's rare and userspace fixes it, it basically
means the answer is: luck of the common case.

But that doesn't help the worst case that can be triggered.

Like I said, if we need to have code to handle that !allow_spinning
case anyway, I'd rather just have a few lines of working code than a
(lengthy) comment explaining the luck of the common case.