Date:   Tue, 11 Jan 2022 12:58:53 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Jordan <daniel.m.jordan@oracle.com>
Cc:     Alexander Duyck <alexanderduyck@fb.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ben Segall <bsegall@google.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ingo Molnar <mingo@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Michal Hocko <mhocko@suse.com>, Nico Pache <npache@redhat.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Steve Sistare <steven.sistare@oracle.com>,
        Tejun Heo <tj@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-mm@kvack.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org
Subject: Re: [RFC 15/16] sched/fair: Account kthread runtime debt for CFS
 bandwidth
Message-ID: <Yd1w/TxTcGk5Ht53@hirez.programming.kicks-ass.net>
References: <20220106004656.126790-1-daniel.m.jordan@oracle.com>
 <20220106004656.126790-16-daniel.m.jordan@oracle.com>
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On Wed, Jan 05, 2022 at 07:46:55PM -0500, Daniel Jordan wrote:
> As before, helpers in multithreaded jobs don't honor the main thread's
> CFS bandwidth limits, which could lead to the group exceeding its quota.
> 
> Fix it by having helpers remote charge their CPU time to the main
> thread's task group.  A helper calls a pair of new interfaces
> cpu_cgroup_remote_begin() and cpu_cgroup_remote_charge() (see function
> header comments) to achieve this.
> 
> This is just supposed to start a discussion, so it's pretty simple.
> Once a kthread has finished a remote charging period with
> cpu_cgroup_remote_charge(), its runtime is subtracted from the target
> task group's runtime (cfs_bandwidth::runtime) and any remainder is saved
> as debt (cfs_bandwidth::debt) to pay off in later periods.
> 
> Remote charging tasks aren't throttled when the group reaches its quota,
> and a task group doesn't run at all until its debt is completely paid,
> but these shortcomings can be addressed if the approach ends up being
> taken.
> 

*groan*... and not a single word on why it wouldn't be much better to
simply move the task into the relevant cgroup..