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Date:   Wed, 2 Nov 2022 09:40:59 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Benjamin Segall <bsegall@google.com>
Cc:     Josh Don <joshdon@google.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v2] sched: async unthrottling for cfs bandwidth
Message-ID: <Y2ItG4BU21Pm0S/u@hirez.programming.kicks-ass.net>
References: <20221026224449.214839-1-joshdon@google.com>
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On Mon, Oct 31, 2022 at 02:56:13PM -0700, Benjamin Segall wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Wed, Oct 26, 2022 at 03:44:49PM -0700, Josh Don wrote:
> >> CFS bandwidth currently distributes new runtime and unthrottles cfs_rq's
> >> inline in an hrtimer callback. Runtime distribution is a per-cpu
> >> operation, and unthrottling is a per-cgroup operation, since a tg walk
> >> is required. On machines with a large number of cpus and large cgroup
> >> hierarchies, this cpus*cgroups work can be too much to do in a single
> >> hrtimer callback: since IRQ are disabled, hard lockups may easily occur.
> >> Specifically, we've found this scalability issue on configurations with
> >> 256 cpus, O(1000) cgroups in the hierarchy being throttled, and high
> >> memory bandwidth usage.
> >> 
> >> To fix this, we can instead unthrottle cfs_rq's asynchronously via a
> >> CSD. Each cpu is responsible for unthrottling itself, thus sharding the
> >> total work more fairly across the system, and avoiding hard lockups.
> >
> > So, TJ has been complaining about us throttling in kernel-space, causing
> > grief when we also happen to hold a mutex or some other resource and has
> > been prodding us to only throttle at the return-to-user boundary.
> >
> > Would this be an opportune moment to do this? That is, what if we
> > replace this CSD with a task_work that's ran on the return-to-user path
> > instead?
> 
> This is unthrottle, not throttle, but it would probably be

Duh..