linux-kernel - Re: [PATCH] psi: Only collect online cpu time in collect_percpu

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Message-ID: <20191206120131.GG2844@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Dec 2019 13:01:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     liu.song11@....com.cn
Cc:     fishland@...yun.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, jiang.xuexin@....com.cn
Subject: Re: [PATCH] psi: Only collect online cpu time in collect_percpu_times

On Fri, Dec 06, 2019 at 12:29:28PM +0800, liu.song11@....com.cn wrote:

> >No, the value will not change, but it need not be 0.
> 
> Hi,
> 
> Suppose there are 4 cpu online, then we take cpu 3 offline. In "collect_percpu_times",
> because "for_each_possible_cpu", will still collect the time of cpu 3 which is offline.

Correct, someone needs to collect the last deltas.

> However, it is clear that "nonidle" will remain at 0 until cpu 3 comes online again.

How does it become 0?

Consider

	CPU2		CPU3

			// runs crap, nonidle increases

	// offline CPU3

			// goes offline, nonidle is still >0


At this point someone needs to collect the delta from CPU3 to make it 0.
But if you only iterate online CPUs, that will not happen.

> And the value of "deltas[s]" will not change after collecting CPU 2. In "get_recent_times",
> the value of "groupc->times_prev[aggregator][s]" corresponding to cpu 3 will not change.

They will not change after the first collection after hot-un-plug. You
need at least one collection after it stops running crap.

> This is only a case where one CPU goes offline. If there are multiple CPUs offline,
> it will correspond to more meaningless operations. So here should be changed to
> "for_each_online_cpu", the effect is exactly the same as "for_each_possible_cpu",
> and the possible meaningless processes are excluded.

It is not. Or if it is, you failed to explain who collects on hotplug.