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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v3 6/9] cgroup/cpuset: Add a new isolated cpus.partition
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To:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>
References: <20210720141834.10624-1-longman@redhat.com>
 <20210720141834.10624-7-longman@redhat.com>
 <20210728160900.GA8905@blackbody.suse.cz>
Message-ID: <f1afbd9e-d16b-c972-c3c0-022a05cec2a6@redhat.com>
Date:   Wed, 28 Jul 2021 12:27:58 -0400
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On 7/28/21 12:09 PM, Michal Koutn? wrote:
> Hello Waiman.
>
> On Tue, Jul 20, 2021 at 10:18:31AM -0400, Waiman Long <longman@redhat.com> wrote:
>> @@ -2026,6 +2036,22 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>> [...]
>> +	} else if (old_prs && new_prs) {
> If an isolated root partition becomes invalid (new_prs == PRS_ERROR)...
>
>> +		/*
>> +		 * A change in load balance state only, no change in cpumasks.
>> +		 */
>> +		update_flag(CS_SCHED_LOAD_BALANCE, cs, (new_prs != PRS_ISOLATED));
> ...this seems to erase information about CS_SCHED_LOAD_BALANCE zeroness.
>
> IOW, if there's an isolated partition that becomes invalid and later
> valid again (a cpu is (re)added), it will be a normal root partition
> without the requested isolation, which is IMO undesired.
>
> I may have overlooked something in broader context but it seems to me
> the invalidity should be saved independently of the root/isolated type.

PRS_ERROR cannot be passed to update_prstate(). For this patchset, 
PRS_ERROR can only be set by changes in hotplug. The current design will 
maintain the set flag (CS_SCHED_LOAD_BALANCE) and use it to decide to 
switch back to PRS_ENABLED or PRS_ISOLATED when the cpus are available 
again.

Cheers,
Longman