linux-kernel - Re: [PATCH] sched/fair: Only increment deadline once on yield

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <7eed0c3d-6a78-4724-b204-a3b99764d839@amazon.com>
Date: Sun, 13 Apr 2025 20:38:00 +0200
From: Alexander Graf <graf@...zon.com>
To: Fernand Sieber <sieberf@...zon.com>, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Vincent Guittot
	<vincent.guittot@...aro.org>, <linux-kernel@...r.kernel.org>,
	<nh-open-source@...zon.com>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: Only increment deadline once on yield


On 01.04.25 14:36, Fernand Sieber wrote:
> If a task yields, the scheduler may decide to pick it again. The task in
> turn may decide to yield immediately or shortly after, leading to a tight
> loop of yields.
>
> If there's another runnable task as this point, the deadline will be
> increased by the slice at each loop. This can cause the deadline to runaway
> pretty quickly, and subsequent elevated run delays later on as the task
> doesn't get picked again. The reason the scheduler can pick the same task
> again and again despite its deadline increasing is because it may be the
> only eligible task at that point.
>
> Fix this by updating the deadline only to one slice ahead.
>
> Note, we might want to consider iterating on the implementation of yield as
> follow up:
> * the yielding task could be forfeiting its remaining slice by
>    incrementing its vruntime correspondingly
> * in case of yield_to the yielding task could be donating its remaining
>    slice to the target task
>
> Signed-off-by: Fernand Sieber <sieberf@...zon.com>


IMHO it's worth noting that this is not a theoretical issue. We have 
seen this in real life: A KVM virtual machine's vCPU which runs into a 
busy guest spin lock calls kvm_vcpu_yield_to() which eventually ends up 
in the yield_task_fair() function. We have seen such spin locks due to 
guest contention rather than host overcommit, which means we go into a 
loop of vCPU execution and spin loop exit, which results in an 
undesirable increase in the vCPU thread's deadline.

Given this impacts real workloads and is a bug present since the 
introduction of EEVDF, I would say it warrants a

Fixes: 147f3efaa24182 ("sched/fair: Implement an EEVDF-like scheduling 
policy")

tag.


Alex