linux-kernel - RE: [RFC PATCH 1/1] sched: Extend cpu idle state for 1ms

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Date:   Fri, 4 Aug 2023 14:04:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Shrikanth Hegde' <sshegde@...ux.vnet.ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "Vincent Guittot" <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Aaron Lu <aaron.lu@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: RE: [RFC PATCH 1/1] sched: Extend cpu idle state for 1ms

From: Shrikanth Hegde
> Sent: 26 July 2023 09:05
...
> > +	if (sched_clock() < READ_ONCE(rq->idle_end_time) + IDLE_CPU_DELAY_NS)
> 
> 
> Wouldn't this hurt the latency badly? Specially on a loaded system with
> a workload that does a lot of wakeup.

Having spotted this I'm also rather worried about systems
that are doing (eg) real time audio and need to wakeup a
lot of threads (less than the number of cpu) every (say) 10ms.

It is hard enough waking up a lot of threads quickly without
another 1ms delay being added.
(I'm only talking about 30 threads as well, not 300.)

	David

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