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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 18 Jan 2023 11:45:45 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linux-RT <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation
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References: <20230117083817.togfwc5cy4g67e5r@techsingularity.net>
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 <20230117165021.t5m7c2d6frbbfzig@techsingularity.net>
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* Mel Gorman <mgorman@techsingularity.net> wrote:

> > > dio_truncate is not a realtime application but indefinite writer 
> > > starvation
> > 
> > If so then the PI boosting would not work if we would have it ;)
> > 
> 
> True, but it's still undesirable for a basic functional test using normal 
> tasks with no prioritisation to stall forever.

Agreed.

> +/*
> + * Allow reader bias with a pending writer for a minimum of 4ms or 1 tick.
> + * This matches RWSEM_WAIT_TIMEOUT for the generic RWSEM implementation.
> + * The granularity is not exact as the lowest bit in rwbase_rt->waiter_timeout
> + * is used to detect recent DL / RT tasks taking a read lock.
> + */
> +#define RWBASE_RT_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
> +
> +static void __sched update_dlrt_reader(struct rwbase_rt *rwb)
> +{
> +	/* No update required if DL / RT tasks already identified. */
> +	if (rwb->waiter_timeout & 1)
> +		return;
> +
> +	/*
> +	 * Record a DL / RT task acquiring the lock for read. This may result
> +	 * in indefinite writer starvation but DL / RT tasks should avoid such
> +	 * behaviour.
> +	 */
> +	if (rt_task(current)) {
> +		struct rt_mutex_base *rtm = &rwb->rtmutex;
> +		unsigned long flags;
> +
> +		raw_spin_lock_irqsave(&rtm->wait_lock, flags);
> +		rwb->waiter_timeout |= 1;
> +		raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> +	}
> +}

So I'm not sure this should be dependent on the task being an RT task.

Starvation scenarios are bad no matter what scheduling policy is used.

Should be unconditional - and all workloads should live with the new 
behavior.

Thanks,

	Ingo