linux-kernel - Re: [PATCH net-next] netfilter: conntrack: Reduce cond_resched frequency in gc

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Message-ID: <aO-ADi4UJhOFz4zr@strlen.de>
Date: Wed, 15 Oct 2025 13:06:01 +0200
From: Florian Westphal <fw@...len.de>
To: "Li,Rongqing" <lirongqing@...du.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"coreteam@...filter.org" <coreteam@...filter.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org
Subject: Re: [PATCH net-next] netfilter: conntrack: Reduce cond_resched
 frequency in gc_worker

Li,Rongqing <lirongqing@...du.com> wrote:

[ CC scheduler experts & drop netfilter maintainers ]

Context: proposed patch
(https://patchwork.ozlabs.org/project/netfilter-devel/patch/20251014115103.2678-1-lirongqing@baidu.com/)
does:

-		cond_resched();
+		if (jiffies - resched_time > msecs_to_jiffies(1)) {
+			cond_resched();
+			resched_time = jiffies;
+		}

... and my knee-jerk reaction was "reject".

But author pointed me at:
commit 271557de7cbfdecb08e89ae1ca74647ceb57224f
xfs: reduce the rate of cond_resched calls inside scrub

So:

Is calling cond_resched() unconditionally while walking hashtable/tree etc.
really discouraged?  I see a truckload of cond_resched() calls in similar
walkers all over networking. I find it hard to believe that conntrack is
somehow special and should call it only once per ms.

If cond_resched() is really so expensive even just for *checking*
(retval 0), then maybe we should only call it for every n-th hash slot?
(every L1_CACHE_BYTES?).

But even in that case it would be good to have a comment or documentation
entry about recommended usage, or better yet, make a variant of
xchk_maybe_relax() available via sched.h...