linux-kernel - Re: [PATCH 11/14] sched/kcpustat: Introduce vtime-aware kcpustat accessor for CPUTIME

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Date:   Thu, 24 Oct 2019 13:50:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 11/14] sched/kcpustat: Introduce vtime-aware kcpustat
 accessor for CPUTIME_SYSTEM

On Wed, Oct 16, 2019 at 04:56:57AM +0200, Frederic Weisbecker wrote:

> +static int kcpustat_field_vtime(u64 *cpustat,
> +				struct vtime *vtime,
> +				enum cpu_usage_stat usage,
> +				int cpu, u64 *val)
> +{
> +	unsigned int seq;
> +	int err;
> +
> +	do {
> +		seq = read_seqcount_begin(&vtime->seqcount);
> +
> +		/*
> +		 * We raced against context switch, fetch the
> +		 * kcpustat task again.
> +		 */
> +		if (vtime->cpu != cpu && vtime->cpu != -1) {
> +			err = -EAGAIN;
> +			continue;

Did that want to be break?

> +		}
> +
> +		/*
> +		 * Two possible things here:
> +		 * 1) We are seeing the scheduling out task (prev) or any past one.
> +		 * 2) We are seeing the scheduling in task (next) but it hasn't
> +		 *    passed though vtime_task_switch() yet so the pending
> +		 *    cputime of the prev task may not be flushed yet.
> +		 *
> +		 * Case 1) is ok but 2) is not. So wait for a safe VTIME state.
> +		 */
> +		if (vtime->state == VTIME_INACTIVE) {
> +			err = -EAGAIN;
> +			continue;

Idem.

If so, you can do return -EAGAIN here, and return 0 at the end and get
rid of err.

Also, if you're spin-waiting here, there should probably be a
cpu_relax() before the return -EAGAIN.

And in case that is so, you probably want the rcu_read_lock() section
below _inside_ the do{}while loop, such that the RCU section doesn't
cover the entire spin-wait.

> +		}
> +
> +		err = 0;
> +
> +		*val = cpustat[usage];
> +
> +		if (vtime->state == VTIME_SYS)
> +			*val += vtime->stime + vtime_delta(vtime);
> +
> +	} while (read_seqcount_retry(&vtime->seqcount, seq));
> +
> +	return err;
> +}
> +
> +u64 kcpustat_field(struct kernel_cpustat *kcpustat,
> +		   enum cpu_usage_stat usage, int cpu)
> +{
> +	u64 val;
> +	int err;
> +	u64 *cpustat = kcpustat->cpustat;
> +
> +	if (!vtime_accounting_enabled_cpu(cpu))
> +		return cpustat[usage];
> +
> +	/* Only support sys vtime for now */
> +	if (usage != CPUTIME_SYSTEM)
> +		return cpustat[usage];
> +
> +	rcu_read_lock();
> +
> +	do {
> +		struct rq *rq = cpu_rq(cpu);
> +		struct task_struct *curr;
> +		struct vtime *vtime;
> +
> +		curr = rcu_dereference(rq->curr);

This is indeed safe now (relies on commit

  5311a98fef7d ("tasks, sched/core: RCUify the assignment of rq->curr")

)

> +		if (WARN_ON_ONCE(!curr)) {
> +			val = cpustat[usage];
> +			break;
> +		}
> +
> +		vtime = &curr->vtime;
> +		err = kcpustat_field_vtime(cpustat, vtime, usage, cpu, &val);
> +	} while (err == -EAGAIN);
> +
> +	rcu_read_unlock();
> +
> +	return val;
> +}
> +
>  #endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */
> -- 
> 2.23.0
>