linux-kernel - Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling.

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Message-ID: <20200828205154.GB29142@worktop.programming.kicks-ass.net>
Date:   Fri, 28 Aug 2020 22:51:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Desfossez <jdesfossez@...italocean.com>
Cc:     Vineeth Pillai <viremana@...ux.microsoft.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Chris Hyser <chris.hyser@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, joel@...lfernandes.org,
        vineeth@...byteword.org, Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        rostedt@...dmis.org, derkling@...gle.com, benbjiang@...cent.com,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Aaron Lu <aaron.lu@...ux.alibaba.com>
Subject: Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and
 scheduling.

On Fri, Aug 28, 2020 at 03:51:09PM -0400, Julien Desfossez wrote:
> +	smt_weight = cpumask_weight(smt_mask);

> +		for_each_cpu_wrap_or(i, smt_mask, cpumask_of(cpu), cpu) {
> +			struct rq *rq_i = cpu_rq(i);
> +			struct task_struct *p;
> +
> +			/*
> +			 * During hotplug online a sibling can be added in
> +			 * the smt_mask * while we are here. If so, we would
> +			 * need to restart selection by resetting all over.
> +			 */
> +			if (unlikely(smt_weight != cpumask_weight(smt_mask)))
> +				goto retry_select;

cpumask_weigt() is fairly expensive, esp. for something that should
'never' happen.

What exactly is the race here?

We'll update the cpu_smt_mask() fairly early in secondary bringup, but
where does it become a problem?

The moment the new thread starts scheduling it'll block on the common
rq->lock and then it'll cycle task_seq and do a new pick.

So where do things go side-ways?

Can we please split out this hotplug 'fix' into a separate patch with a
coherent changelog.